INDIANAPOLIS (AP) — In one of the most conservative states in the nation, supporters of gay marriage are pondering the unthinkable: a victory, or at least not a loss.

A proposal to amend Indiana's constitution to ban same-sex marriage has sparked a flurry of phone banks and appeals to big-money donors as the state prepares to become a 2014 battleground on an issue that has largely been decided in other states.

Indiana is one of just four states that ban gay marriage in statute only; 29 others have constitutional bans. But none of the other states with statutory bans — Pennsylvania, West Virginia and Wyoming — face the pressure in place in Indiana, where lawmakers must approve a proposed ban and send it to voters in November unless they want to restart the process from scratch.

That the issue's fate is even in question is remarkable in Indiana, which in recent years has become a model of conservative causes ranging from school vouchers to right to work. In 2011, state lawmakers overwhelmingly voted in favor of the amendment in the first of two required votes, and with Republican supermajorities in both legislative chambers, its final passage seemed a slam-dunk.

But the tides have shifted. Voters in Maryland, Maine, Washington and Illinois have approved gay marriage, and polls have shown increasing numbers of Indiana voters oppose a constitutional ban even though most still oppose gay marriage.

"Everyone else in the country is moving toward more equality. Indiana is kind of the last stand of folks that are trying to put something like this into their constitution," said Megan Robertson, a veteran Indiana Republican operative tapped to manage Freedom Indiana, a bipartisan coalition working to block the ban.

Opponents have argued for years that the constitutional ban is unnecessary and will paint the state as intolerant and hurt businesses' efforts to recruit top talent. They're especially concerned about a provision in the proposed amendment that also bans civil unions and employee benefits for same-sex couples.

Volunteers with Freedom Indiana are staffing nightly phone banks and calling lawmakers who supported the amendment the first time in hopes of changing their minds before the Legislature reconvenes next month. Top companies including drugmaker Eli Lilly & Co. and engine-maker Cummins Inc. have contributed $100,000 each to the campaign. And a recent fundraiser featuring Mary Cheney, the daughter of former Vice President Dick Cheney who has been a vocal supporter of same-sex marriage, was sponsored by some of the state's top GOP money men, including the campaign finance chairman for Republican Gov. Mike Pence.

Phil Cooper, a 63-year-old retired bus driver from Bloomington whose adult daughter has sometimes identified as a lesbian, said he has been making phone calls for Freedom Indiana two to three times each week since September.

"It really, really troubles me to see her being singled out because of that single characteristic," he said of his daughter.

He said he is "cautiously optimistic" about blocking the amendment and said getting more information out about its effects, including the ban on employer benefits for same-sex pairs, has helped turn the momentum.

At least two lawmakers who voted for the amendment in 2011 have said they will oppose it next year. Senate Appropriations Chairman Luke Kenley, R-Noblesville, said last year that placing the ban in the constitution would not be a "productive" use of time for state lawmakers. And state Rep. Sean Eberhart, R-Shelbyville, told The Shelbyville News last month that he made a mistake in supporting the amendment last time and "to put that amendment in the constitution and to lock down generations with bigotry is wrong."

Senate President Pro Tem David Long, R-Fort Wayne, said he still supports the marriage ban but has been listening to his two sons, who oppose the measure. And while House Speaker Brian Bosma, R-Indianapolis, says he expects a vote by lawmakers on the issue next year, he notably left the issue out of the House Republican Caucus' 2014 legislative agenda.

Supporters of the amendment, who have distributed fliers about the issue to churches, contend a constitutional ban would prevent future lawmakers from changing the law. They say Freedom Indiana's business argument is a scare tactic and point to reports showing top job growth coming mostly in states that already have constitutional bans on gay marriage.

Even if national attitudes on the issue have changed, they say, Indiana residents still firmly oppose gay marriage and should be allowed to weigh in.

"The future of marriage belongs in the hands of voters," said Micah Clark, executive director of American Family Association of Indiana.

Pence, well-known for his social and religious conservatism, says he supports traditional marriage but has largely stayed on the sidelines.

The outcome of the debate could hinge on which side winds up with the most clout: the fledgling Freedom Indiana group, with the weight — and money — of corporate Indiana behind it, or the supporters, who have decades of lobbying experience among them and deep ties in the Statehouse to trade on.

"We don't have a half-million dollars to pour into this, like the other side does," Clark acknowledged.

The pressure Indiana Republicans feel makes sense, given that they likely face the most peril in how the issue plays out. If the amendment fails in the Legislature, incumbent Republicans could face the wrath of conservatives in the May primaries. And if the issue makes it to the ballot, it has the potential to rev up the Democratic base in November's general election in a way that could ripple up and down the ticket.

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  • New York

    New York lawmakers <a href="http://www.huffingtonpost.com/2011/07/24/new-york-gay-marriage_n_907901.html" target="_blank">legalized same-sex marriage on July 24, 2011</a>, making it the largest state at the time to pass such legislation.

  • Maryland

    Voters in Maryland <a href="http://www.huffingtonpost.com/2012/11/07/gay-marriage-victory_n_2085900.html" target="_blank">approved marriage equality in the November 2012 election</a>. Initially, the gay marriage bill was signed into law by Gov. Martin O'Malley (D) on March 1, 2012, but opponents gathered enough signatures to force the issue back onto the ballot. With the passing of marriage equality, same-sex marriage <a href="http://www.huffingtonpost.com/2012/12/31/maryland-gay-marriage_n_2389044.html" target="_blank">ceremonies began on Jan. 1, 2013</a>.

  • Connecticut

    Connecticut's Supreme Court <a href="http://www.huffingtonpost.com/2008/10/10/connecticut-gay-marriage_n_133605.html" target="_blank">ruled that same-sex couples had the right to marry on Nov. 12, 2008</a>, making it the third state in the nation to do achieve marriage equality.

  • Iowa

    Iowa's Supreme Court <a href="http://www.huffingtonpost.com/2009/04/03/iowa-gay-marriage-ban-rul_n_182782.html" target="_blank">ruled the state's gay marriage ban unconstitutional</a> on April 3, 2009.

  • Maine

    Maine <a href="http://www.huffingtonpost.com/2012/11/07/gay-marriage-victory_n_2085900.html" target="_blank">made history in the November 2012 election</a> when it became the first state to pass marriage equality on the ballot. Human Rights Campaign President Chad Griffin said, "Voters in Maine came to the common-sense conclusion that all people deserve the ability to make loving, lifelong commitments through marriage." Just three years ago, a popular vote overturned legislation that would have legalized same-sex marriage in the state.

  • Massachusetts

    Massachusetts became the first state in the nation to <a href="http://www.history.com/this-day-in-history/first-legal-same-sex-marriage-performed-in-massachusetts" target="_blank"> legalize same-sex marriage on May 17, 2004</a>. The state's Supreme Court initially found the ban on gay marriage unconstitutional on Nov. 18, 2003.

  • New Hampshire

    Same-sex couples were able to <a href="http://www.cbsnews.com/2100-201_162-6042937.html" target="_blank">begin seeking marriage licenses</a> on Jan. 1, 2010.

  • Vermont

    Vermont, which invented civil unions, became <a href="http://www.huffingtonpost.com/2009/04/07/vermont-legalizes-gay-mar_n_184034.html" target="_blank">the first state to legalize gay marriage through a legislature's vote</a> -- overriding the governor's veto. Same-sex couples were able to begin marrying on Sept, 1, 2009.

  • Washington D.C.

    Gay couples were able to <a href="http://www.huffingtonpost.com/2009/12/01/gay-marriage-dc-council-p_n_375435.html" target="_blank">begin marrying in the nation's capital</a> on March 9, 2010.

  • California

    The state initially began conducting gay marriages on June 16, 2008. On November 5, 2008, however, California <a href="http://www.huffingtonpost.com/2013/06/26/proposition-8-timeline_n_3503512.html" target="_blank">voters passed Proposition 8</a>, which amended the state's constitution to declare marriage as only between a man and a woman. On June 26, 2013, by a 5-4 vote, <a href="http://www.huffingtonpost.com/2013/06/26/hollingsworth-v-perry-ruling_n_3438269.html" target="_blank">the Supreme Court justices held in Hollingsworth v. Perry</a> that the traditional marriage activists who put Proposition 8 on California ballots in 2008 did not have the constitutional authority, or standing, to defend the law in federal courts after the state refused to appeal its loss at trial, opening the door for marriages to resume in the state.

  • Washington

    On February 13, 2012, Gov. Christine Gregoire (D) <a href="http://www.huffingtonpost.com/2012/02/13/washington-gay-marriage-signed-chris-gregoire_n_1273887.html" target="_blank">signed a law allowing same-sex marriage ceremonies</a> to begin on June 7, 2012. The process was delayed by gay marriage opponents who gathered enough signatures to put the issue up to a state vote in November 2012. <a href="http://www.huffingtonpost.com/2012/12/09/washington-gay-marriage-law_n_2266574.html" target="_blank">Gay marriage passed on November 7, 2012.</a> The official determination for Washington did not come until one day after the election because of the state's mail-in voting system.

  • Rhode Island

    Gay marriage came to Rhode Island when Governor Lincoln Chafee <a href="http://www.huffingtonpost.com/2013/08/01/gay-marriage-minnesota-rhode-island_n_3686034.html" target="_blank">signed the marriage equality bill</a> into law on May 2, 2013.

  • Delaware

    Delaware obtained gay marriage when <a href="http://www.huffingtonpost.com/2013/05/07/delaware-gay-marriage-law-_n_3232771.html" target="_blank">Governor Jack Markell signed the marriage equality bill it into law</a> on May 7, 2013.

  • Minnesota

    Minnesota same-sex couples achieved marriage equality when Gov. Mark Dayton signed the legislation into law <a href="http://www.huffingtonpost.com/2013/05/14/minnesota-gay-marriage-legal-_n_3275484.html" target="_blank">on May 14, 2013</a>.

  • New Jersey

    Newark Mayor Cory Booker <a href="http://www.huffingtonpost.com/2013/10/20/cory-booker-same-sex-marriage_n_4134116.html?&ir=Gay%20Voices&utm_hp_ref=gay-voices" target="_blank">began marrying same-sex couples</a> at City Hall at 12:01 a.m. on Oct. 21, 2013.

  • Hawaii

    Hawaii Gov. Neil Abercrombie signed same-sex marriage into law on Nov. 13, 2013, making it the 15th state to pass such legislation.

  • Illinois

    Illinois became the 16th state to legalize gay marriage, with the House <a href="http://www.huffingtonpost.com/2013/11/05/illinois-gay-marriage_n_4220793.html" target="_blank">having passed the bill on Nov. 5</a>. and Gov. Pat Quinn signing the legislation on Nov. 20.

  • New Mexico

    On Dec. 19, the New Mexico Supreme Court <a href="http://www.huffingtonpost.com/2013/12/19/new-mexico-gay-marriage_n_4474507.html?ir=Gay%20Voices" target="_blank">unanimously ruled</a> that same-sex marriage rights are protected under the Constitution.