Sen. John McCain canceled a fundraiser for Republican Brian Bilbray in California's special election after the candidate publicly criticized the Senate immigration bill.

McCain, R-Ariz., the top proponent of comprehensive immigration reform, didn't appear at the Wednesday breakfast fundraiser.

Bilbray is in a June 6 run-off election with Democrat Francine Busby to replace the vacant seat of embattled former Rep. Randy "Duke" Cunningham, who was sentenced to eight years and four months in prison for accepting bribes.

Bilbray has spent hundreds of thousands of dollars on TV commercials attacking Busby for her support of the immigration bill that McCain co-authored with Massachusetts Democratic Sen. Edward Kennedy.

"Perhaps Senator McCain canceled his appearance after learning that lobbyist Brian Bilbray has spent hundreds of thousands of dollars attacking the McCain immigration reform plan that Francine supports," said Brennan Bilberry, a spokesman for Busby.

Busby's campaign released an online commercial to promote her support of McCain's plan.

"I was looking forward to Senator McCain's visit to San Diego so he could explain his tough immigration reform plan to San Diegans," Busby said. "It is disappointing that Brian Bilbray does not want to have a serious conversation about Senator McCain's tough immigration reform that I support."

California Republican Rep. David Dreier was to replace McCain, FOX News has learned. Dreier's presence is aimed at identifying Bilbray with the congressman's efforts to lead House efforts on lobbying reform. A Bilbray campaign aide told FOX News that the Bilbray camp considered McCain's scheduled appearance potentially harmful.

Other Republicans have voiced opposition to the Senate immigration bill. Rep. James Sensenbrenner, R-Wis., chairman of the House Judiciary Committee, campaigned for Bilbray and criticized a previous version of the Senate bill.

Republicans want to replace Cunningham's seat with one of their own as some consider it to be a test of the Democrats ability to capitalize on President Bush's low job approval poll numbers.

Republican National Committee Chairman Ken Mehlman has deployed 70 field staffers to canvass voters and generate turnout for Tuesday's election. The Democrats are sending House Minority Leader Nancy Pelosi to campaign for Busby. California's two Democratic senators, Dianne Feinstein and Barbara Boxer, will also campaign for Busby.

The Democratic congressional committee on Tuesday sent out a mass fundraising e-mail by chairman Rep. Rahm Emanuel.

"The years when Democrats just played defense are over. This election, it will be Democrats who set the tone forcing Republicans to frantically spend millions of dollars to defend themselves. It will be Democrats who go on to Republican turf and make them fight for their political lives," according to Emanuel's message.

Cunningham won two years ago 58 percent to 36 percent over Busby. The district voted 55 percent for Bush and 44 percent for Democratic nominee John Kerry. Bush defeated former Vice President Al Gore in 2000 with 54 percent to 43 percent.

FOX News' Major Garrett contributed to this report.