Richard Riordan brought his struggling GOP gubernatorial campaign to the conservative central coast on Thursday, as Bill Simon kept up his attacks on the former front-runner four days before the primary.

Riordan toured downtown San Luis Obispo on foot and in an open-air electric car, pressing his candidacy and jesting with a noontime crowd in this farm-and-college town.

"You know, you could be executed for not voting," the moderate former Los Angeles mayor joked to one 18-year-old who said he wasn't registered.

Seeking to cut into Simon's new standing in polls as the top choice of conservative voters, Riordan repeated criticisms of his opponent for not voting in some elections and failing to register as a Republican until 1992.

"He's only been a Republican a third of his adult life," Riordan said of the 50-year-old businessman.

Simon, meanwhile, pressed his anti-tax message in Sacramento, where he touted his support for a proposed 2004 ballot measure to increase tax credits for California homeowners and renters.

Secretary of State Bill Jones also continued to traverse California in his six-seat Cessna on Thursday, trying to boost a campaign that polls show is far behind that of Simon and Riordan.

Simon, who once trailed Riordan by a large margin, has climbed dramatically in recent weeks and was the leading Republican candidate in a statewide poll released Wednesday.

The Field Institute poll showed 37 percent of likely GOP voters supporting Simon, 31 percent in favor of Riordan and 9 percent for Jones. It had a margin of error of plus or minus 5 percentage points.

During Simon's rise, Riordan has been the target of attack ads by Democratic Gov. Gray Davis questioning his honesty and has had to defend his past contributions to Democratic candidates and his position on abortion and other issues.

As the race has tightened, Riordan and Simon, who are personal friends, have abandoned what had been a cordial campaign, taking aim at each other in commercials and on the campaign trail.

On Thursday, the two traded fresh charges, as both sought to lay claim to their party's conservative base.

Riordan supporters, including Assemblywoman Lynn Daucher, a Brea Republican, accused Simon of ignoring issues that are important to GOP women, while Riordan blasted Simon's $1,000 donation last year to city attorney candidate Mike Feuer, who supported strict gun control laws.

Simon also was forced to explain a $10,000 donation supporting a 2000 measure aimed at making it easier to pass school bonds. The measure was opposed by the anti-tax groups that now support Simon.

"I probably wouldn't do it again," Simon said, "but having said it, I did it because I saw that there was an emergency need for schools."

Simon's campaign also distributed a 1998 article reporting that Riordan supported extending a temporary sanitation tax in Los Angeles; Riordan said he had no recollection of the issue.

The attacks likely will intensify as the candidates enter the campaign's final weekend. Simon and Riordan plan to continue to saturate television airwaves with commercials, and Simon has a packed travel schedule that will take him across the state.

Though Riordan has ground to make up in polls, his public schedule is lighter, and he plans to honor an agreement he made a year ago to make two appearances with a San Francisco comedy troupe.