California voters turned out in low numbers on Tuesday for a primary that could pave the way for a shakeup of the state's congressional delegation following election rule changes and an overhaul of the state's political district boundaries.

While new maps and new rules may have changed the way the game of California politics is played, making some districts more competitive across party lines, early official estimates of statewide turnout hovered at just over 15 percent.

Under California's new open primary rules, the two top vote getters in each district will proceed to the general election in November regardless of party, so two Democrats or two Republicans could be competing for the same seat come Nov. 6.

Redistricting has also tossed some longtime incumbents into the ring against each other after a decade of remarkable stability in the state's majority Democratic delegation in the House of Representatives.

In one of the most closely watched races, where two Democratic incumbents appeared poised to face off against one another in the general election, just a trickle of voters cast ballots on a sunny day in California's 30th district in Los Angeles County, according to early results.

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Among those casting ballots there were 82-year-old Ester Berrenson and her husband of 64 years, 86-year-old Manny, who walked to their polling station during the breezy afternoon. Both voted for Democratic incumbent Brad Sherman against fellow Democratic incumbent Howard Berman.

"He comes out and talks to us," Manny Berrenson said, explaining why he and his wife had voted for Sherman. "He goes to the high school at least half a dozen times a year. He goes there and talks about what he's doing."

Due to the "top two" rule, both are likely to advance to a final competition in November, according to early returns. Sherman had 40.3 percent to Berman's 32.6 percent just after midnight with 18 percent of precincts reporting.

In their newly redrawn district, about 50 percent of voters there come from Sherman's old stomping ground, while just 25 percent come from Berman's prior district.

"Tonight is far from over, but we're confident about our momentum moving into the general election," a senior adviser to Berman's campaign, Brandon Hall, said in a statement.

Changes favor Democrats

The changes in California are generally seen to favor Democrats in a state that gave President Barack Obama a 24-point margin of victory over his Republican rival John McCain in 2008, analysts said.

Two incumbent Democratic congresswomen, Janice Hahn and Laura Richardson, also appeared set to face off against each other in November in the 44th district in Los Angeles County in a race in which no Republicans were running.

Shortly after midnight, Hahn was leading with 62.1 percent with 6.5 percent of precincts reporting.

For years, political stability in the nation's most populous state 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.

But ahead of this election, a non-partisan citizen's commission was put in charge of 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 non-partisan 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.

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

Democrats made progress in one key U.S. House race where analysts had said they risked being frozen out of the general election should a Republican and a right-leaning independent candidate advance to the November polls.

But, as early returns trickled in, Democratic state Assemblywoman Julia Brownley appeared poised to face a Republican challenger, state Senator Tony Strickland, in November in California's coastal 26th district.

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 existing seats.

Nationwide battle

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 nation's hottest with the state's only House member, Republican Denny Rehberg, 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.

In Iowa, State Representative Pat Grassley - the grandson of U.S. Senator Chuck Grassley - won his primary, defeating another Republican in the Iowa House in a contest forced upon him and State Representative Annette Sweeney by redistricting.

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

Republican Heather Wilson and Democrat Martin Heinrich are projected to win their respective party races and run against each other for the coveted Bingaman seat, according to Brian Sanderoff, president of Research and Polling Inc in Albuquerque.

In New Jersey, polls are closed in the vote to fill the seat of U.S. Representative Donald Payne, the state's first black congressman, who died in March. His son is among the candidates.

CUSTER, SD -- South Dakota Sen. John Thune said he's open to serving as Mitt Romney's running mate, telling reporters in his home state on Wednesday that it would be tough to ever rule out that option.

"I don’t think you ever rule out or say no to opportunities to public service if you are really interested in making a difference for your country, you want to put your gifts and abilities to the highest and best use,” Thune told reporters following a town hall in the local firehouse here. But, he added, "I don’t expect that to happen but I don’t think you never say never when it comes to serving your country."

Thune seems to have shifted in the way he's spoken about about his contact with the Romney campaign. Just two weeks ago, the senator told The Hill, he had yet to be contacted by the Romney campaign. But today, Thune said, “We talk to him all the time.”

Other than knowing who is leading the search, Romney’s vice presidential selection process has been rather secretive. Thune’s comments come on the same day one of Romney’s top advisers, John Sununu, gave National Reviewone of the first glimpses into the grueling process. Sununu said Thune is “on the list for consideration” for VP.

The small town senator, who even flirted with running for president himself this cycle, seems happy with his current position as the third-ranking Republican in the U.S. Senate.

Being vice president for Thune, he says, is “not a job I aspire to.”

"I like the job I have. I look forward to working with a president, as a member of the United States Senate, that is interested in solving problems," he said. "I think that Gov. Romney and his team will make a very good choice. I think, some of the names I am hearing banging about out there are really good people."

Sources say the senator may soon hit the campaign trail for Romney, perhaps in neighboring Iowa. Thune endorsed Romney in Des Moines, Iowa just before Thanksgiving -– weeks before many other prominent Republicans picked a candidate.

“I came out and supported him [Romney] early because I thought he…represented our best opportunity to win in November and also the guy who was best experienced and had the best skills to govern our country for the next few years, which is not going to be easy,” Thune said, standing in front of a Custer fire truck the day after Romney secured the required 1,144 delegates for the party’s nomination. “I am delighted he has crossed that line and is going to be our nominee because I think it is going to be a great race this fall.”