By Mary Slosson

SACRAMENTO, June 5 (Reuters) - California voters head to the polls on Tuesday in a primary contest set to launch the biggest political scramble in the state in at least a decade following the redrawing of U.S. Congressional boundaries and election rule changes.

The changes could set the stage for head-to-head face-offs between longtime incumbents, potentially from the same party, in November after a decade of remarkable stability in the state's majority Democratic delegation in the House of Representatives.

That stability was a result of the deliberate creation of electoral districts to favor incumbents, a process known as gerrymandering. In 263 elections from 2002 to 2010, only one congressional seat changed political party.

"California was totally locked in on this gerrymandered map," said Kyle Kondik, political analyst at the Center for Politics at the University of Virginia.

This time the state put a non-partisan citizens commission in charge of most of the redrawing of congressional districts. The number of districts did not change, but the boundaries were adjusted to reflect population shifts since the previous national census in 2000.

California will also see the two candidates who get the most votes advance to the general election in November, regardless of party. This means that two Democrats or two Republicans could be competing for the same congressional seat on Nov. 6.

"With nonpartisan redistricting and this new 'top two' primary system, California suddenly becomes very interesting and one of the more competitive states in the entire country," Kondik said.

A dozen districts that had once been predictable are now in play, according to California Republican Party Chairman Tom Del Beccaro, resulting in heavy spending as candidates, some of whom had to move into their new districts, introduce themselves to new constituents.

"This is going to be a record year for campaign spending in California," Del Beccaro said. "There's going to be a dramatic rise in spending, not only for this primary but also this fall."

For first time since 1920, slowing population growth meant California did not add any seats to its 53-member congressional delegation, adding even more uncertainty by matching incumbents against one another in some redrawn districts.

"Short term, this is off the rails; this is crazy," said Paul Mitchell, a Democratic consultant in California.


INCUMBENT VS. INCUMBENT

Redistricting has created a "huge leap" in the number of districts with more than 50 percent minority voters, Mitchell said. Majority-Hispanic legislative districts have increased from 19 to 29, and California now has the only majority-Asian district in the continental United States.

Two congressional contests have attracted particular attention with two longtime Democratic incumbents running against each other.

Representatives Howard Berman and Brad Sherman are duking it out in a closely watched race in California's 30th district, in Los Angeles County. Due to the "top two" rule, both are likely to advance to a real competition in November, analysts said.

"This primary on Tuesday for them is like a pre-season NFL football game," Kondik said. "It's like a dress rehearsal for the actual election."

The same is true of two incumbent Democratic congresswomen, Janice Hahn and Laura Richardson, who are facing off in the new 44th district in Los Angeles County.

In their quest to win back a majority in the U.S. House, Democrats would have to gain four or five seats in California, which Kondik said would be difficult but not impossible.

Republicans control the House with a 242-190 majority, with three seats vacant. Outside California and Illinois, Democrats are mainly playing defense, trying to hold seats they already have.


NATIONWIDE BATTLE

Several other states also hold primaries on Tuesday.

In Montana, the U.S. Senate battle is one of the hottest in the nation. Montana's only member of the House, Republican Denny Rehberg, is challenging first-term Democratic Senator Jon Tester.

Both candidates are known across the state and are expected to easily win their respective party primaries on Tuesday.

Republicans are targeting a New Mexico U.S. Senate seat vacated by retiring Democratic Senator Jeff Bingaman in their quest to win a U.S. Senate majority in 2012. Democrats hold a 51-47 majority, with two independents who usually vote with Democrats.

While New Mexico leans Democratic because of its large Hispanic population, the state occasionally elects Republicans, including Governor Susana Martinez.

Former Congresswoman Heather Wilson is expected to easily win the Republican nomination over businessman Greg Sowards, said Brian Sanderoff, president of Research and Polling Inc in Albuquerque.

The Democratic Senate primary race is expected to be more competitive, with Congressman Martin Heinrich against state Auditor Hector Balderas. While Heinrich leads in the polls, Sanderoff said undecided Hispanic voters will most likely choose a Hispanic surname when they go to vote.

In New Jersey, primary voters will select a replacement for U.S. Representative Donald Payne, the state's first black congressman, who died in March.

Payne's son, Newark City Council President Donald Payne Jr., is one of six candidates. The elder Payne had represented New Jersey's 10th congressional district since 1989.

Primaries are also to be held in South Dakota, North Dakota, and Iowa, but the most competitive races in those states will be in November. (Reporting by Mary Slosson; Additional reporting by Dan Boyce in Montana, Zelie Pollon in New Mexico, David Bailey in Minnesota, Kay Henderson in Iowa and Edith Honan in New York; Editing by Lisa Shumaker)

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