Fractures among conservatives in the affluent coastal communities extending north of San Diego coupled with dissatisfaction with President Bush have put Democrats within striking distance of capturing a safe Republican seat that was thrown open when Representative Randy Cunningham resigned after pleading guilty to corruption charges.
Though Mr. Bush carried the district, the oceanfront 50th, by 10 points in 2004 and Republicans have a 44 percent to 29 percent edge in voter registration, polls show Brian P. Bilbray, a Republican, and Francine Busby, a Democrat, essentially tied going into Tuesday's special election, which each party is desperate to win.
"It is going to be close," Mr. Bilbray, a former congressman, acknowledged. The campaign will have cost an almost-unheard-of $10 million when spending by the candidates, the national parties and others is totaled.
The election is serving as a laboratory to assess the potency of political appeals that are likely to resonate through November. Mr. Bilbray, noting that the Mexican border is just miles away, is running as a fierce foe of illegal immigration, while he and national Republicans portray Ms. Busby as an advocate of higher taxes and government benefits for those who entered the United States illegally. The ballot lists Mr. Bilbray's occupation as immigration reform consultant.
Ms. Busby, a local school board member who lost decisively to Mr. Cunningham in 2004, has joined national Democrats in pounding Mr. Bilbray for working as a lobbyist after serving in Congress. They have sought to tie him to what Democrats denounce as the institutional corruption defined by Mr. Cunningham's illegal efforts to steer military projects to the firms of lobbyists who rewarded him with payoffs.
"This is ground zero for people who have felt the cost of corruption," said Ms. Busby, who said the sense of betrayal that Republican voters felt over Mr. Cunningham's deeds would win her support. "We are going to see if the people in this district are going to rise up above partisanship and send a message that it is time for a change."
Many analysts describe Ms. Busby as a solid but hardly overwhelming candidate who, in an ordinary year, would have no chance of winning such a Republican-tilted district. But this year, Democrats are energized while some Republicans turned off by Republican spending and uncertain about the course of the nation under Mr. Bush are not rallying to their party's candidates.
"I think the whole atmosphere emanating from national politics is really what is keeping her afloat," said Gary Jacobson, a political scientist at the University of California, San Diego. "Democrats are really unhappy and really motivated to get out and vote, and Republicans are not."
Democrats and others say that if a Democratic candidate can pose a legitimate threat in a district like this one, challengers will have even better opportunities in races where the party's edge is much narrower.
"They have to run in quite a few districts in November that are a lot worse for them," said Stuart Rothenberg, a nonpartisan Washington-based analyst of Congressional races. "While a Busby win is ominous for Republicans, even if Bilbray squeaks out a win, it is hard for me to see how Republicans can take that as good news."
Ms. Busby is not Mr. Bilbray's only obstacle in the race, which has featured a barrage of negative television commercials by both parties. He is also under fire from a fellow Republican, Bill Hauf, a wealthy real estate investor who is challenging Mr. Bilbray in a separate but simultaneous primary race for a spot on the November ballot to fill the 50th District seat for a full term.
Mr. Hauf has poured some of his money into mailings to Republicans questioning Mr. Bilbray's conservative commitment, an effort that has infuriated local Republican leaders who say the feud could sap critical support from Mr. Bilbray.
"I have come to be the fly in the ointment," said Mr. Hauf, who said he and like-minded Republicans in the district saw Mr. Bilbray as too moderate and tied to the Republican establishment to help turn around a party they view as addicted to higher spending and soft on social issues.
Complicating matters further, Mr. Bilbray is also opposed in the race to fill the seat through the end of the year by William Griffith, an independent who received the endorsement of prominent local anti-immigration leaders, potentially undermining him on his central issue.
Hoping to exacerbate that split, Ms. Busby's campaign on Friday took the unusual step of broadcasting advertisements on conservative radio stations highlighting Mr. Griffith's endorsements in effect running an advertisement for an opponent in an effort to weaken Mr. Bilbray. The Democratic Congressional Campaign Committee mailed similar material on Mr. Griffith to Republicans.
Mr. Bilbray said such tactics were deplorable. But he said voter turnout was a genuine concern given Republican frustration with Mr. Bush and the Senate for backing an immigration plan that could lead to citizenship for some illegal immigrants.
"The outrage against the president and the Senate's proposal is so strong that it could cause collateral damage for us," he said.
National Republican officials say they expect that Mr. Bilbray will pull out a victory on Tuesday. They say that they do not see the race as a portent for November and that his struggles are not unusual given the unpredictable nature of special elections. In the closing days, the campaigns were exchanging last-minute attacks with most of their efforts and those of the national parties devoted to identifying voters and encouraging them to get to the polls.
Both of the leading contenders to represent a district that is home to some of the best-known surfing beaches in the nation keep surfboards in their campaign headquarters and share an enthusiasm for the sport, which gave Mr. Bilbray an alternative idea for settling the issue.
"If the election is real close," he said, "maybe we can just have a surf-off."
