(CNN) -- GOP front-runner Sen. John McCain picked up an endorsement Monday from former New York Gov. George Pataki, providing a last-minute boost before Super Tuesday.
Sen. John McCain addresses supporters Monday at a stop in Boston, Massachusetts.
McCain also has won the support of Arnold Schwarzenegger, governor of delegate-rich California.
Those endorsements could play a role in Tuesday's vote, as the support of Florida Gov. Charlie Crist did in his state's primary.
According to exit polls, more than 40 percent of Florida primary voters indicated Crist's decision to back McCain had an effect on their vote, and those voters went to McCain overwhelmingly.
Recent polls show McCain with a comfortable edge heading into Tuesday's 21 Republican contests.
Rival Mitt Romney spent his final day of campaigning trying to chip away at McCain's lead by attacking his conservative credentials.
Romney, the former Massachusetts governor, has been telling voters he is the conservative alternative to McCain.
Romney has been trying to tap into anger at McCain, reminding voters the Arizona senator voted against tax cuts and supported campaign finance reform.
"We disagree on a number of issues, and if you look at Sen. McCain's position on a number of issues, you have a hard time distinguishing him from Hillary Clinton and Barack Obama," he said Monday.
Romney told supporters in Georgia he is "definitely the underdog compared to Sen. McCain."
"But the nice thing about being the underdog is that we typically win," he said.
Romney, along with former Arkansas Gov. Mike Huckabee, is campaigning in the Southeast before making a last-minute stop in California, where McCain and Romney are neck-and-neck.
Romney Sunday suggested Huckabee get out of the race because he was splitting the conservative vote. Watch why Romney thinks Huckabee should quit »
"The truth of the matter is -- he's a good man. Everybody has every right to stay in this race until the very end," Romney said on CNN's "Late Edition."
"We all battled in Florida. Sen. McCain and I came in No. 1 and No. 2, very close, and he came in a distant fourth. I think by virtue of that, I think most people around the country have said, OK, it's been narrowed down to a two-person race."
Huckabee shot back Monday on CNN's "American Morning," disputing the notion he was siphoning off Romney voters. Watch Huckabee describe what's at stake for him on Tuesday »
"He suggested that I get out of the race and has the audacity to say that the reason is because the voters that I'm getting are voters that would go to him," Huckabee said.
"I think it's incredibly presumptuous and even arrogant to suggest that the voters who are voting for me would automatically gravitate to him. I think they, quite frankly, would not."
Huckabee has been trying to get his voice heard, reminding voters that this is not a two-man race as he campaigns throughout the South, with stops in Alabama, Tennessee and his home state of Arkansas.
Huckabee has accused Romney of engaging in voter suppression, but Romney said that's not "the right word."
"It's not voter suppression. I want people to vote, but I want them to vote for me," Romney said while campaigning in Atlanta, Georgia. "First, a couple of rules in politics. One - no whining, and No. 2 - You get them to vote for you, and so I want them not to vote for Mike Huckabee and not to vote for John McCain and to vote for me."
In his final hours of campaigning, McCain was darting across the Northeast, making stops in Romney's backyard. He spent Monday morning in Massachusetts -- where Romney was governor for four years. See which states are up for grabs »
McCain has been attacking Romney for raising taxes, while refuting claims he's not conservative enough.
"I want to tell you, and look you in the eye, my friend, as president of the United States, I will preserve my proud conservative Republican credentials, but I will reach across the aisle to the Democrats and work together for the good of this country.
"That's what you want us to do," McCain said at a rally in Boston, Massachusetts.
Polls suggest McCain has large leads in New York, New Jersey, Connecticut and Illinois, but he's only ahead by single digits in Georgia and Missouri.
The key to many races is undecided voters, which could make up from 10 to 15 percent of those who cast ballots.
The Republicans have 1,020 delegates at stake in the 15 primaries and six caucuses Tuesday.
To become the Republican nominee, a candidate must win 1,191 delegates. See why the delegates matter
So far, McCain leads in delegates with 97. He's followed by Romney with 92, Huckabee with 29 and Texas Rep. Ron Paul with six.
Huckabee won Iowa's first-in-the-nation caucuses, but has yet to take any other states. McCain won New Hampshire, South Carolina and Florida, and Romney finished first in Maine, Nevada, Michigan and Wyoming.
The CNN/Opinion Research Corp. national poll results, released Monday, show McCain with the support of 44 percent of registered Republicans. Romney came in second with 29 percent.
Huckabee had 18 percent, followed by Paul with 6 percent.
The poll, conducted February 1-3, has a margin of error of plus or minus 5 percentage points.
CNN's "poll of polls" of the Republican race puts McCain at 45 percent compared with 24 percent for Romney.
The poll of polls is an average of five national polls conducted in February. Those five surveys were done by CNN/Opinion Research Corp., Gallup, Pew, ABC and CBS. E-mail to a friend
CNN's Paul Steinhauser, Rebecca Sinderbrand and Dana Bash contributed to this report.