When San Diego voters go to the polls Tuesday in a special election to replace disgraced former Mayor Bob Filner, they will be seeing one odd choice on the ballot: Nathan Fletcher, the one-time hope of the Republican pundit class, is a contender to become mayor of America’s Finest City — but he’s running as a Democrat.

Filner stepped down Aug. 30 after 20 women stepped forward to accuse him of being a serial sexual predator. Filner’s ouster — and the former 20-year Democratic congressman’s subsequent plea deal to a felony false-imprisonment charge — don’t seem to be haunting his party.

Instead, the Democrats in the field appear certain to gather 55 percent or more of the vote, setting up an early 2014 runoff pitting the top Democrat against City Councilman Kevin Faulconer, 46, an affable, pro-business Republican who seems certain to make the next round. A poll released Sunday by the U-T San Diego newspaper and KGTV-TV 10 News showed Faulconer with 40 percent of the vote, consistent with other surveys over the past month.

The battle to be the Democrat in the runoff appears tight. The poll released Sunday had Qualcomm executive Nathan Fletcher ahead of City Councilman David Alvarez 24 percent to 22 percent.

The two are strikingly different. Alvarez, 33, a San Diego State honors graduate and the product of a rough city neighborhood, is a prototypical urban California Latino Democrat, devoted to such causes as a “livable wage” for hotel workers and union-favoring agreements on both private and public construction projects.

Like Filner, Alvarez is a populist critic of a San Diego status quo that has long been perceived as pro-downtown business interests despite a strong Democratic edge in voter registration.

Fletcher, 36, was once a Republican wunderkind — a telegenic war hero with a smooth TV manner married to a former Bush 43 staffer who won election to the Assembly representing wealthy La Jolla in 2008. He seemed designed by computer for success as a California Republican. Republican commentators routinely described Fletcher as a rising star in the troubled state party.

But after four years as an assemblyman with a slight maverick streak — he spoke up for gays in the military, sought to close a dubious corporate tax break and pushed for some criminal-justice reforms — Fletcher couldn’t win over the local GOP establishment in early 2012 when he ran for mayor.

The party instead backed the actual maverick Carl DeMaio, a councilman who helped downsize city government and reduce retirement benefits for city employees.

This led Fletcher to quit the Republican Party and run as an independent, a move trumpeted by New York Times columnist David Brooks in a puff piece that praised Fletcher’s “can-do pragmatism” while lamenting the party “insurgents” who backed DeMaio, whom Brooks glossed as a “orthodox conservative” — though in fact DeMaio is a gay libertarian who supports legal abortion and same-sex marriage.

Fletcher’s gamble didn’t pay off. With both Democratic and Republican groups paying for ads depicting Fletcher as an opportunist, DeMaio finished first in the June 2012 mayoral primary and Filner second. Filner went on to win 53 percent to 47 percent in November’s runoff, becoming San Diego’s first Democratic mayor in 20 years.

In May 2013, Fletcher’s political evolution continued when he announced he was joining the Democratic Party. Even though just 14 months before, Fletcher appeared to relish union-bashing, his debut as a Democrat was greeted with remarkable union support, led by Assemblywoman Lorena Gonzalez, a San Diego Democrat and former CEO of the local labor coalition.

When the race to replace Filner began in September, Fletcher won the endorsement of most government unions, while Alvarez dominated among private sector unions. Fletcher also has the backing of Gov. Jerry Brown, several local Democratic elected officials and wealthy biotech entrepreneurs. But Alvarez still has most of the trappings of the Filner coalition, led by the strong support of interim Mayor Todd Gloria and San Diego’s most beloved Democrat, former Councilwoman Donna Frye, who first exposed Filner’s sexual harassment in an emotional July press conference.

Yet after a campaign with only one major gaffe — Fletcher’s repeated false claim to be the only member of his family who had gone to college — Alvarez still appeared to head into the final days of the campaign in shaky shape, his campaign suffering from an unexpected weakness: tepid support from Latinos, who make up 29 percent of San Diego’s 1.3 million residents. The poll released Sunday showed only 34 percent of Latino voters wanted him to become the first Latino elected as mayor.

This reflects both Fletcher’s significant union support and the lingering bad feelings in the local Latino political establishment over Alvarez’s 2010 decision to run against Felipe Hueso for City Council, blocking Hueso’s expected election to the council seat being vacated by his brother, Ben Hueso, who is now a state senator.

Faulconer will likely be the favorite over either Fletcher or Alvarez in the runoff. Most strategists assume he’ll hold the 47 percent of the vote won by DeMaio in November 2012 and do better among moderates put off by DeMaio’s hard edge and among social conservatives who were uncomfortable with the idea of a gay mayor.

But in a city with a 90,000-plus Democratic voter-registration edge, a win by traditional Democrat Alvarez wouldn’t seem like much of a surprise.

Nor would a win by Fletcher — except, perhaps, to the many local Republicans who long expected him to be on their side when he took over at City Hall.

Reed is an editorial writer for the U-T San Diego newspaper and a regular contributor to CalWatchdog.com.