It's not about his stance on the issues, but California's 15th District is shaping up to give Rep. Pete Stark (D) his biggest challenge in 39 years of office.

The state's new open primary system plus redistricting has resulted in tomorrow's primary pitting the 19-term Stark against an up-and-coming Democrat. Both are expected to advance to the general election, under California's new "top-two" rules that allow the first and second finishers onto the November ballot, regardless of party.

Neither side is releasing polls, but outside observers expect 31-year-old Eric Swalwell (D), an Alameda County prosecutor, to be competitive.

Swalwell has come out swinging, attacking the 80-year-old Stark for trying to claim his Maryland home as his primary residence, missing votes and being passed over for chairmanship of the Ways and Means Committee in 2010 despite being the most senior committee member.

"The outcome, percentagewise, in June is going to be really indicative of how serious the challenge is against Stark in November," said Al Pross, an editor of the nonpartisan political almanac California Target Book. "To the extent Stark has more than a 10 percent advantage, based upon the primary, if it's 10 or less, then I would think you'd have to say he's in real trouble."

Aside from age, the candidates' main difference appears to be partisanship. At a labor union rally in Swalwell's hometown of Dublin yesterday, Stark emphasized the danger of losing an experienced Democrat as Republicans launch attacks on health care and Social Security. "We need to protect Social Security and Medicare because that's what Democrats are all about," he said. "To have experience is urgent."

By contrast, Swalwell says he will work with Republicans. "I'm not a Washington, D.C., entrenched incumbent, but I'm ready to go back there and work across the aisle," he said in an interview. "I'm ready to work with anyone who's willing to work with me."

While President Obama endorsed Stark last month, Swalwell has also sought to establish a link.

"I believe our campaign is modeled very much on his campaign in 2008, running as a Washington outsider against entrenched incumbents, against institutional Washington," he said in a May 23 radio interview.

Stark has raised about $565,000, to Swalwell's $300,000. But Stark still has about $400,000 of it on hand, while Swalwell is down to $45,000. Both sides have employed mailings, phone calls and door-to-door visits; neither has bought any radio or TV spots. About half of the district is new to Stark, but about 35 percent of registered voters are 55 or older. About half are Democrats, with the rest split between Republicans and those that decline to state a party.

"Eric and Stark are ideologically pretty similar," Swalwell campaign spokeswoman Lisa Tucker said. "It's coming down to the fact that Stark doesn't live in the district, has one of the worst voting records in Congress and is not very attentive to his district."

Stark, for his part, has been even more acerbic. "I have a son who's much older than this pipsqueak, who's been a district attorney for 25 years," Stark said in an April debate, the candidates' only direct debate so far. "Would you all go to a surgeon to have your heart transplanted who's never done a heart transplant?"

But Stark has also made a number of public gaffes that local media have seized upon, including falsely accusing Swalwell of taking bribes from developers and confusing bankrupt solar company Solyndra -- in his hometown of Fremont -- with Tesla Motors.

The San Francisco Chronicle endorsed Swalwell last month, citing Stark's missteps as well as Swalwell's support for the technology industry and the district's national laboratories, Lawrence Livermore and Sandia national labs.

"His performance in this campaign has produced cringe-worthy moments that show a mean-spiritedness and recklessness with the facts -- and a detachment from one of the region's critical issues -- that suggest it is time for change," the paper's editorial board wrote of Stark last month.

The race's third candidate, tea party-backed Chris Pareja, 40, who points out that he was "born the year Pete was first elected," is skeptical of the concept of green jobs. He thinks the Bay Area, and California at large, is too expensive to house manufacturing.

"Instead of a scenario like Solyndra, where $535 million goes to one company, having a $50 million carrot out there could potentially launch dozens of companies," he said in an interview.

Swalwell disagrees, saying he supports the "intent behind the funding" of Solyndra but thinks there should have been more oversight. He also is in favor of more stimulus funding for energy and green jobs. "We are a country that's been struggling to think big, and I think we need to reinvent ourselves when it comes to green energy," he said.

He is less bullish on regulating greenhouse gases, while Stark introduced a bill last year that would impose a tax of $10 per ton of emissions. "I do want to go back there and work to make our carbon footprint smaller, but I am very mindful that doing that can also chill jobs," Swalwell said. "I know Senator Cantwell has a cap-and-dividend program; that's an interesting program."

Once the primary ends, the candidates are likely to zero in on the areas where they got the most support, Pross said. "The battleground is going to be in November, for Stark to be able to protect the percentage he gets in the primary, and for Swalwell to really analyze those precincts where he won and try and increase his vote there," he said.