SACRAMENTO (Reuters) - California voted on Tuesday in a primary contest set to launch the biggest political scramble in the state in at least a decade following adoption of an open primary system and the redrawing of U.S. Congressional district boundaries.
With all candidates appearing on a single ballot, the two with the most votes will advance to the general election in November regardless of their party, so two Democrats or two Republicans could be competing for the same seat on November 6.
In addition, redistricting will set the stage for head-to-head face-offs between some longtime incumbents after a decade of remarkable stability in the state's majority Democratic delegation in the House of Representatives.
For years 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.
This time a non-partisan citizens commission was charged with redrawing congressional districts. The number of districts did not change, but the boundaries were adjusted to reflect population shifts since the last national census in 2000.
"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," said Kyle Kondik, political analyst at the Center for Politics at the University of Virginia.
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."
Redistricting has created a "huge leap" in the number of districts with more than 50 percent minority voters, said Paul Mitchell, a Democratic consultant. Majority-Hispanic 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."
In the 30th district at a polling place in the Los Angeles suburb of Reseda on Tuesday morning, a few senior citizens and people on their way to work trickled in to cast their ballots.
Manny Berrenson, 86, said he voted for Sherman and the "big difference" in the race was that Berman seemed to advertise more. "In other words, there was more money behind him than Sherman," Berrenson said. "I didn't like that."
In a similar race, two incumbent Democratic congresswomen, Janice Hahn and Laura Richardson, 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.
Primaries were also held on Tuesday in New Jersey, New Mexico, Montana, South Dakota, North Dakota and Iowa.
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.
Additional reporting by Dan Boyce in Montana, Zelie Pollon in New Mexico, R.T. Watson in California; Editing by Anthony Boadle