The leading gubernatorial candidates have spent a lot of time in South Jersey, but the race probably will be decided in the shadow of the George Washington Bridge.

With 530,000 registered voters, Bergen County has more voters than any county in the state, and is a must-win for a Republican statewide candidate. A Democrat can finish second there, as Gov. Corzine did in his 2000 U.S. Senate run, and still win the state - but it has to be a narrow margin.

"There is no other county in the state with that kind of concentration of voters, let alone that concentration of Democratic voters," said Mike Kasparian, Bergen County Democratic chairman.

Carl Golden, who worked for both recently successful Republican governors, Thomas H. Kean and Christie Whitman, said, "Republicans have to win it [Bergen] to offset the kinds of pluralities Democrats roll up in places like Hudson and Essex Counties, and throw Camden in on top of it."

The county's GOP chairman, Robert Yudin, said flatly, "Christie can't win unless he takes Bergen."

The three major campaigns have been working hard in Bergen, swinging governor and lieutenant governor candidates through diners, lawyers' breakfasts, ribbon cuttings, and senior centers, in places such as Fort Lee, Garfield, Saddle Brook, and Hackensack.

Last night, Corzine's running mate, State Sen. Loretta Weinberg, who hails from Teaneck in Bergen, took the Democratic National Committee chairman, Virginia Gov. Tim Kaine, to a union rally in Saddle Brook.

Today, Christie plans to bring in former New York Mayor Rudy Giuliani for stops in North Bergen and Paramus. New York mayors are about as well-known in North Jersey as Philadelphia mayors are in South Jersey.

Volunteers for both major parties have been calling voters for weeks in Bergen and have made lists of whom to call - and not call - on Election Day.

Republicans have added staffers to the county party machine, and received national party help as well, said state chairman Jay Webber, a Morris County assemblyman.

Independent Chris Daggett also has cast a hopeful eye toward Bergen; he thinks he can do well there because his voters might be able to find him on the ballot. Only in Bergen and Gloucester Counties is Daggett located near the major-party candidates on voting machines. Elsewhere, he is placed behind several minor gubernatorial candidates.

In the last decade, Bergen has flipped from a Republican county to a reliable Democratic county. Only one Republican, County Clerk Kathy Donovan, won countywide office in 2008.

Yudin sees a silver lining in the near wipeout.

"Last year, for the first time in six or seven years, we became competitive again," he said.

In last year's presidential race, Democrat Barack Obama swept the county with 225,367 votes, a 39,249 edge over Republican John McCain. On the same ballot, Yudin noted that "Republican freeholders were only losing by 8,000 [votes] while Obama was winning by 40,000."

He said the GOP has recently taken back a few municipal seats but conceded "the days of winning in Bergen by huge pluralities are gone for good, but there's no reason we have to lose. We can be competitive."

That is especially true, he said, in the close race between Corzine and Christie.

Democrats outnumber Republicans there, 169,000 to 111,200. But the biggest voting group, as it is everywhere else in the state, consists of unaffiliated voters, who number 250,300. Unaffiliated voters have recently been voting with the Democrats.

Kasparian, Bergen's Democratic chairman, said, "For the last 10 years, more Democratic votes have been garnered from Bergen than anywhere else."

To help work Bergen, Corzine has used his running mate Weinberg, who has long been active in Democratic politics, to campaign there. He also has appeared there with Obama.

Weinberg also serves another political purpose for Corzine. The county's former Democratic chairman, Joseph Ferriero, was convicted last week on corruption charges. When Republican Christie has noted that Corzine helped fund Ferriero's political machine, Corzine fires the Weinberg bullet.

Weinberg fought with Ferriero for years, pushing back his attempt to undermine her Senate candidacy four years ago. She also sponsored some of the state's tougher ethics and campaign-finance laws.

Kasparian, who replaced Ferriero, said that while he felt some trepidation when he stepped into the job last January, he has been pleasantly surprised that the county party has pulled together.

"Everybody is motivated to a Democratic cause," he said. "The failure of one or two people does not equate to the party as a whole."

Contact staff writer Cynthia Burton at 856-779-38558 or cburton@phillynews.com.