At stake are 52 Republican delegates.
Giuliani, who until recently held a commanding lead in the Garden State, is expected to endorse McCain, his friendliest rival, after the former New York City mayor finished a distant third in Florida's primary.
New Jersey voters who supported Giuliani are expected to support McCain as well, considered to be the most moderate of the remaining Republican candidates, according to pollster Patrick Murray."We have a very moderate electorate in New Jersey, and people like Giuliani and McCain are perfect fits for that kind of electorate," said Murray, director of the Monmouth University Polling Institute.
Neither of the candidates support a constitutional amendment banning gay marriage, and both favor relaxing restrictions on federal funding for stem cell research. They disagree on abortion rights, with Giuliani in support and McCain opposed.
Most New Jersey voters find Mitt Romney and Mike Huckabee too conservative, Murray said.
However, Sen. Joe Kyrillos, chairman of Romney's New Jersey effort, cautioned against assuming that a majority of Giuliani supporters would now back McCain.
"There was a collapse of Rudy support around America before yesterday, so there is not that much of it to go around," said Kyrillos. "There are many rank and file Republicans who are uncomfortable with Sen. McCain, including people previously committed to Rudy Giuliani."
He said he expected at least some of those people to gravitate to Romney.
On Wednesday, Rep. Peter King, a New York Republican and adviser to Giuliani, raised the prospect of Giuliani remaining on the campaign trail - for McCain.
"I expect him to be fairly active for McCain. There is a real friendship and respect between the two," he said.
McCain, who has rocketed to front-runner status in recent weeks, was scheduled to be at a Hamilton firehouse Monday afternoon, the day before New Jersey and 21 other states hold primaries of caucuses.
McCain's New Jersey campaign chairman, Sen. Bill Baroni, said he expected New Jersey Republicans would be unified behind McCain.
"We believe that Mayor Giuliani and Sen. McCain had a lot of people who liked both of them, supported both of them and share a commitment to a strong American foreign policy," Baroni said. "They have a very good relationship, and we're looking forward to bringing our whole party together to support Sen. McCain."
Giuliani had enjoyed a large lead in polls taken before the primaries and caucuses began.
However, in the most recent Quinnipiac University poll taken earlier this month, McCain actually edged past Giuliani in New Jersey, though the margin was too small to be considered statistically significant.
Murray said Giuliani's endorsement of the Arizona senator is huge.
It means McCain can assume widespread support in former Giuliani states like New Jersey, New York and California, and can focus his limited financial resources on the Midwest and the South, he said.
Democrat John Edwards also pulled out of the presidential race Wednesday.
Sen. President Richard Codey, Edwards' biggest New Jersey backer, is backing Obama.
Codey is set to make his choice official at a 1 p.m. news conference on Thursday in West Orange, according to an official involved in planning the announcement. The official spoke only on condition of anonymity so as not to upstage Codey.
Gov. Jon S. Corzine, an early backer of Sen. Hillary Rodham Clinton, said he hoped Edwards' backers would now support Clinton.
"We would like to see anyone who had previously been in the Edwards camp come into the Clinton support group," Corzine said Wednesday, "But they're going to have to make those choices themselves."
Key New Jersey Democrats are split in their support. Corzine, U.S. Sen. Robert Menendez and Assembly Speaker Joe Roberts are among Clinton's backers, while Rep. Steve Rothman, Newark Mayor Cory Booker and Sen. Loretta Weinberg support Obama.