The third time for Tom Campbell wasn't a charm, but a strike.

His latest failed bid for the U.S. Senate left the former congressman from Silicon Valley believing he is unelectable to statewide office in California, where the primary system is stacked against the kind of middle-of-the-road candidate he represents.

Campbell stumbled on all the Republican defining issues — abortion, guns, gay marriage and taxes — as he lost the GOP nomination to former Hewlett-Packard Co. chief executive Carly Fiorina.

"He's come up short three times, and after awhile the people begin to look at those kinds of things as damaged goods," San Jose State University political science professor Larry Gerston said Wednesday.

Campbell lost the GOP primary in 1992 to Bruce Herschensohn, who was defeated by then-congresswoman Barbara Boxer. He won the Republican nomination in 2000, but lost to Democratic Sen. Dianne Feinstein, the incumbent, in the general election.

Voters' rejection of Campbell, a soft-spoken business professor with a law degree and doctorate in economics, illustrates a larger problem for Republican candidates running for statewide office in California: They must run as fiscal and social conservatives to win their party's nomination, but then quickly change tactics to win a general election. Less than a third of registered voters in California are Republicans.

After his latest loss, Campbell wondered if even Abraham Lincoln and 19th century statesmen Daniel Webster and Henry Clay, known as the great compromisers, could win election in today's harsh partisan environment. He worried that voters are turning from veteran politicians like himself just as their experience is needed to heal the economy and fight unconventional wars.

"There's a broader trend in our country, and it's a dangerous trend," Campbell said in a telephone interview Wednesday.

California voters' approval of Proposition 14 offers some hope, he said. The ballot measure will let voters cast ballots for any candidate in a primary, regardless of party, with the top two moving on to the general election.

"It has the possibility at the statewide level of electing people who are willing to work together," Campbell said.

Campbell, 57, would not say if he will try again for elective office. He was packing for a week of fishing in Canada, then plans to resume a quiet life outside the political spotlight, teaching politics and business at Chapman University in Orange this fall and at the University of California, Berkeley, next spring.

He might have a chance running in a congressional district, which is the path suggested by California Republican Party Chairman Ron Nehring. He encouraged Campbell to run for a seat currently held by a Democrat.

Campbell has difficulty in Republican primaries because his social views are the opposite of most Republican voters. He favors abortion rights and gay marriage, and has supported state tax increases in the past. He alone among the three Republicans in the U.S. Senate race opposed letting people on the federal government's "no-fly" list buy guns.

Campbell said polling showed voters were willing to put those issues aside if they knew of Campbell's experience with business and the economy. Ironically, polls consistently showed he was the Republican candidate most likely to defeat Boxer in a general election matchup.

"It's at precisely at a time like this, when you have these unprecedented challenges, that our political system should bring forward individuals who are prepared to work with each other instead of individuals who are polarized," Campbell said as he conceded the race.