Interstate 8 has long been a political divide in San Diego and maybe never more than in the campaign to replace Mayor Bob Filner, who resigned amid a torrent of sexual harassment allegations.

Republican Kevin Faulconer, a city councilman, easily topped a field of 11 candidates in a first round of voting by dominating in newer, wealthier neighborhoods north of the freeway. Democrat David Alvarez, also a councilman, finished second by cleaning up south of the freeway in more densely populated, predominantly Latino areas.

If Faulconer prevails in Tuesday's runoff, San Diego would be the nation's largest city with a Republican mayor. He would be the only Republican to lead a major city in California, where Democrats hold all statewide offices.

If Alvarez wins, he would be the first Latino mayor in the city's 164-year history. To overcome the 18-point gap between him and Faulconer in the first round, he must have a strong showing south of Interstate 8 among voters who made Filner the city's first Democratic leader in 20 years in 2012.

Tom Shepard, a longtime strategist who ran campaigns for both parties, said the north-south divide has shaped San Diego politics for as long as he can remember. Republicans solidified their base north of the freeway as bedroom communities sprang up in recent decades.

"The only difference over the last 10 years is that voters south of the 8 are very informed, organized labor is turning them out and they're capitalizing on their numbers better than they used to," said Shepard, who is not involved in the race.

Latinos make up 29 percent of the city's 1.3 million residents. The San Diego County registrar of voters says they account for 18 percent of the electorate.

The National University System Institute for Policy Research says 60 percent of votes in the first round were cast north of Interstate 8, and turnout there was significantly higher than it was south of the freeway. The registrar estimates Tuesday's overall turnout around 45 percent.

Mail-in ballots are expected to favor Faulconer, so Election Day turnout is crucial for Alvarez.

Alvarez, 33, has relentlessly attacked Faulconer as a shill for corporate interests. Faulconer, 47, portrays Alvarez as a tool of labor unions.

Despite sharp ideological differences, few issues have separated the candidates.

Both promise more attention to neighborhood priorities like street repairs, library hours and emergency response times, putting less emphasis on ambitious civic projects like expanding the convention center and bringing a new stadium for the NFL's Chargers.

Filner, 71, embraced the same "neighborhoods-first" mantra but Faulconer and Alvarez scarcely mention the disgraced former mayor, who pleaded guilty in October to one felony count of false imprisonment and two misdemeanor counts of battery.

The former 10-term congressman began a three-month sentence of home confinement on Jan. 1.

Alvarez and the super PACs supporting him — largely from organized labor — have raised $4.5 million, according to an analysis of campaign filings by nonprofit news website newsinewsource.org. That compares to $3.5 million raised by Faulconer and committees backing him, largely business groups.

One mailer from a pro-Alvarez union ridiculed Faulconer for membership in the San Diego Yacht Club. A labor-backed television ad scrolled the names of companies whose executives backed Faulconer, suggesting the Republican is in lock step with business.

A mailer from the pro-Faulconer, pro-business Lincoln Club shows an altered photo of Alvarez flashing what critics have said is a gang sign. Another group tells voters in a mailer that Alvarez fought to direct federal grants to three low-income areas, saying he neglected others and suggesting he would govern the nation's eighth-largest city the same way.

Alvarez, who grew up speaking Spanish at home, extols his family's immigrant roots at almost every campaign stop.

The San Diego native introduced himself at a forum geared toward Asian voters by saying his parents left Mexico with third-grade educations to raise a family in the hardscrabble area that he now represents. His father was a janitor for 26 years and his mother washed dishes at fast-food restaurants.

He was the first to finish college among his siblings, some of whom dabbled in gangs but went on to get good jobs and own homes.

The first-term councilman then laid out his populist campaign theme of stripping power from the "hoteliers and developers who have controlled the city for such a long time and are used to having things the same way."

Faulconer touches lightly on his personal history, which includes a term as student body president at San Diego State University and a career as a public relations executive. Instead, he reminds the crowd of the fiscal turmoil that enveloped the city when he joined the council eight years ago, drawing a contrast with his less experienced opponent.

Faulconer plays down his party affiliation, casting himself as a consensus-builder, in a city where Democrats have built a 13-percentage-point edge over Republicans in voter registration. "I'm not a bomb-thrower," he said.

Faulconer quickly acknowledged that neighborhoods north of the freeway have had an advantage, bolstered by developer fees on new homes that pay for parks and other services.

"We need to do a much better job in neighborhoods south of Interstate 8," he said at news conference in a weed-infested lot to tout a plan he calls 'One San Diego.' "We've seen this work in other areas. We need to have leadership to make it work in every single neighborhood."

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