Rep. Mike Honda, a seven-term House member who represents the overwhelmingly Democratic South Bay, hasn't had to debate an opponent since 2000.

But he's just one of the candidates who may have to brush up on his skills before June because of increasing political pressure to confront challengers in a more competitive California arena.

Honda, 72, is facing the fight of his political career in his Silicon Valley district from fellow Democrat and former Obama administration trade representative Ro Khanna, 37, who has amassed a 4-1 cash-on-hand advantage.

Khanna is demanding the chance to debate the San Jose congressman. Honda's team hasn't ruled out a debate, but it hasn't agreed, either.

In past years, the Democratic incumbent in a comfortable Democratic district would never have had to worry about a challenge, let alone a debate. But as the Honda-Khanna race shows, new election rules are shaking up old practices.

Under a primary system that took effect for House races in 2012 and applies in statewide contests for the first time this year, the top two vote-getters in the June primary advance to the November general election, no matter their party affiliation. The resulting pressure for debates isn't limited to Democrats.

Republican contest

In the governor's race, Assemblyman Tim Donnelly, R-Twin Peaks (San Bernardino County), a Tea Party favorite, has been pushing former Treasury Department official Neel Kashkari to debate at this weekend's Republican state convention in Burlingame. Both are also itching to debate Gov. Jerry Brown, should they make it into a top-two spot. Some experts say it may be difficult under the new system for incumbents to dodge a debate.

"The conventional wisdom has always been that the candidate in the lead has fewer reasons to participate," said Shanto Iyengar, a political science professor and director of the political communication laboratory at Stanford University. But that may not be practical in the age of social media, when candidates are reaching out to voters practically on a 24/7 basis, he said.

"Debates are very much critical learning opportunities for large numbers of voters," Iyengar said.

'Practical decision'

Thomas Hollihan, a communications professor at the University of Southern California, said campaign staffs normally make "a very practical decision: Is my candidate likely to do well, and is it likely to help or hurt?"

Today, with YouTube available to preserve stumbles forever, the stakes are higher than ever.

East Bay Rep. Pete Stark, a Democrat who had served 20 terms in Congress, was considered all but invulnerable until he faced a 2012 "top two" challenge from Eric Swalwell, a largely unknown prosecutor and fellow Democrat. In a disastrous debate, Stark accused Swalwell of taking bribes, an allegation he was later forced to retract as groundless, and Swalwell went on to end the congressman's political career.

But Hollihan said there's another important consideration for campaigns: "Can I hide from the debate and not suffer consequences?"

It worked in 2012 for Sen. Dianne Feinstein, D-Calif., who refused to debate her GOP opponent, Danville activist Elizabeth Emken.

"People know my views. I don't mince words," said Feinstein, who dismissed Emken as "somebody who got 12 percent in the primary and who has no experience."

Tough spot for Honda

But while Feinstein took little heat for refusing to debate a political neophyte, Honda may not have that luxury.

"He's in a fairly tough race," Iyengar said, noting that Khanna's endorsers include such high-profile Silicon Valley names as Facebook's chief operating officer, Sheryl Sandberg, and Yahoo CEO Marissa Mayer.

"If I were to choose, I'd opt for a debate," Iyengar said. "He (Honda) can always show off what he knows."

Republican strategist Rob Stutzman said, "Honda has to worry about his opponent, so my guess is he'll want to debate - unless he wants to be defined as someone who runs away from it."

Khanna doesn't appear ready to let Honda off the hook. He has brought in Jeff Bleich, former U.S. ambassador to Australia and onetime head of the San Francisco Bar Association, to work with media organizations on sponsoring such a debate.

Honda spokesman Vivek Kembaiyan said last week, "We certainly haven't ruled out debates."

Growing list

Complicating matters is the growing number of Republican candidates who have jumped into the race for Honda's seat. They include Stanford Hospital physician Vanila Singh, 43 of Fremont; tech recruiting executive Joel Vanlandingham, 47, of San Jose; and Google attorney Vinesh Singh Rathore, 35, of San Jose.

The nonpartisan League of Women Voters has scheduled an hourlong candidates' forum for May 3 at Fremont City Hall and says Honda, Khanna and the Republican hopefuls are all expected to be there.

But candidates' forums, preferred by campaigns, are not the same as debates, experts say.

Forums typically feature prescreened questions and allow little or no opportunity for candidates to challenge each other directly. Debates typically require candidates to take questions from reporters, address each other directly and answer follow-ups.

Carla Marinucci is senior political writer for The San Francisco Chronicle. E-mail: cmarinucci@sfchronicle.com Twitter: @cmarinucci