SEATTLE, Oct 25 (Reuters) - President Barack Obama on Thursday endorsed state ballot initiatives to legalize same-sex marriage in Washington state, Maryland and Maine as he sought to galvanize gay and lesbian enthusiasm for his re-election bid.

Statements issued by the Obama campaign in those three states urging voters to approve the gay-marriage measures on their respective Nov. 6 ballots came five months after Obama became the first U.S. president to express support for the right of same-sex couples to wed.

With polls showing Obama and Republican challenger Mitt Romney running neck and neck, the outcome of the race is likely to hinge largely on the Election Day turnout of their core constituencies, and both candidates are doing the utmost to motivate their supporters.

For Obama, the gay and lesbian community is seen as a loyal and potentially pivotal voting bloc, along with larger demographic groups such as Hispanics and single, young women.

"While the president does not weigh in on every single ballot measure in every state, the president believes in treating everyone fairly and equally, with dignity and respect," according to a statement in Seattle by Paul Bell, press secretary for Obama's campaign in Washington and Oregon.

The state goes on to explicitly urge a yes vote for Washington state Referendum 74, which would allow same-sex couples to marry.

Nearly identical endorsement statements were issued in support of similar initiatives on the Nov. 6 ballot in Maine and Maryland.

Six states and the District of Columbia have already extended marriage rights to gay and lesbian couples, all brought about by court rulings and legislative action.

Maine, which rejected gay marriage in a 2009 referendum by 53 percent to 47 percent, could become the first state to legalize same-sex marriage solely by popular vote.

In Washington state and Maryland, whose legislatures passed laws expanding marriage rights to gay and lesbian couples earlier this year, citizens will vote on whether to let those statutes stand.

Opponents of gay marriage hope Minnesota will become the latest state to pass a constitutional amendment defining marriage as exclusively a union between a man and a woman - effectively banning same-sex nuptials.

Obama previously spoke out against the Minnesota measure.

In Washington state, where polls show the "yes" campaign for gay marriage running ahead of the opposition in a tightening race, campaign manager Zach Silk said in a statement that "having the president weight in on approving Referendum 74 puts an extra gust of wind in our sails."

A recent Washington Post poll found voters in Maryland likely to approve the state's gay marriage law in the November election.

Brian Ellner, co-founder of The Four 2012 campaign, which is fighting for gay marriage rights in Washington, Maryland, Maine and Minnesota, said Obama's support "has had a big impact on public opinion, especially among African-Americans," adding that his endorsement could be decisive in a close election. (Additional reporting by Steve Gorman and Edith Honan; Writing by Steve Gorman; Editing by Peter Cooney)

Related on HuffPost:

Loading Slideshow...
  • Connecticut

    Since November 12, 2008

  • Delaware

    Gay marriage law <a href="http://www.huffingtonpost.com/2013/05/07/delaware-gay-marriage-law-_n_3232771.html" target="_blank">enacted</a>, weddings to begin July 1.

  • Iowa

    Since April 3, 2009

  • Maine

    In 2012, Maine voted in favor of a ballot amendment to legalize gay marriage.

  • Maryland

    The gay marriage bill was signed into law by Gov. Martin O'Malley (D) on March 1, 2012. Opponents later gathered enough signatures to force the issue back onto the ballot in November 2012, but voters rejected the effort against gay marriage.

  • Massachusetts

    Since May 17, 2004

  • Minnesota

    Same-sex marriage bill signed into law in May. Gay marriages will begin in August.

  • New Hampshire

    Since January 1, 2010

  • New York

    Since July 24, 2011

  • Rhode Island

    Bill passed in May. Law takes effect on August 1, 2013.

  • Vermont

    Since September 1, 2009

  • Washington

    On February 13, 2012, Gov. Christine Gregoire (D) signed a law allowing same-sex marriage ceremonies 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. They voted to approve it on Election Day.

  • Washington D.C.

    Since March 9, 2010

  • California

    The state initially began conducting gay marriages on June 16, 2008. On November 5, 2008, however, California voters passed Proposition 8, which amended the state's constitution to declare marriage as only between a man and a woman. In 2013, the Supreme Court ruled against that law, and the state shortly thereafter began sanctioning same-sex nuptials.