Why are white voters crossing party lines?

It has been 20 years since a majority of white city voters pulled the lever for a Democratic mayoral candidate — even as most remain registered Democrats themselves, the latest election results show.

According to a Pace University exit poll, Mayor Michael Bloomberg won last Tuesday among whites with 67 percent to 30 percent for Fernando Ferrer.

As expected, Ferrer won — by smaller margins — among blacks and Latinos.

The difference in voting by race was stark across the city in the 1989, 1993 and 1997 elections, when Rudolph Giuliani was the Republican-Liberal candidate.

Giuliani won the last two of those contests. When in 1989 Democrat David Dinkins beat him to become the first African-American mayor, it was on the tide of a record turnout of black voters. When Dinkins lost four years later, it came after the Crown Heights riots that further polarized the city along racial lines.

In chronological order, the four losing Democrats over 12 years represented an ethnic mix: one black male, one Jewish female, one Jewish male and one Latino male.

Mark Green, the Democrat defeated by Bloomberg in the razor-close 2001 race, said that without nonwhite voters, even Democrat Bill Clinton would not have won the White House in 1992 or 1996.

“It is true nationally,” Green said. “And it is true in New York City.”

“Why is that? While it is un-P.C. to say it, my best guess is that some — not most, but some [white voters] — are sufficiently threatened by minorities that, without acknowledging it, they tilt toward the party that’s overwhelmingly white, rather than diverse,” Green said.

While racial and ethnic voting numbers are imprecise, Mickey Blum, president of the Blum and Weprin Associates polling firm, said the working estimate is that about half the city’s voters these days are white.

“Perhaps in the last couple of elections, the white Jewish and white Catholic vote have looked very much the same, and that is not always true,” she said. “It may not be true in future contests, but in the last few it has been the case.”

For example, Italian voters have long been considered more conservative as a group and likely to vote GOP. “The Jewish vote is still more Democratic,” Blum said, “but perhaps in an election like this, where people do not think of Bloomberg as really Republican, some Jews probably cast their first Republican votes.”

There are about 3.9 million registered voters in the five boroughs, according to city Board of Elections figures. That includes 2.6 million Democrats, and estimates conclude nearly half of them are white, said Marist poll director Lee Miringoff. There are 477,169 Republicans, the figures show, most of them white.