"It's with some nostalgia that I return to this place that I love so well," McCain said during a town meeting in Manchester, N.H., "but I am really here for one reason only and that is to make sure than we make Mitt Romney the next president of the United States of America."
McCain, who won the New Hampshire primary in 2000 and 2008, predicted The Granite State would "catapult" Romney to victory.
Before he left Des Moines, Iowa, Wednesday, Romney said he would have to brush up on Rick Santorum's positions after the former U.S. senator from Pennsylvania virtually matched him in the Iowa caucuses.
"… We really haven't had much opportunity to get to know Rick Santorum on the issues," Romney told MSNBC. "And so as time goes on here, we'll get a better chance to see where we agree and where we disagree."
Unofficial results from Iowa's Tuesday caucuses had Romney, a former Massachusetts governor, finishing eight votes ahead of Santorum with each receiving about a quarter of the votes cast in the first-of-the-nation Republican nominating process.
U.S. Rep. Ron Paul of Texas was third about 3 percentage points back.
"Maybe these upcoming debates will give us a better sense of that," Romney said of Santorum. "But frankly over the past several months, our effort has been focused on contrasting and comparing with [former House] Speaker [Newt] Gingrich, with Rick Perry, with Herman Cain."
Gingrich, who finished a distant fourth in Iowa, said he intends to hit Romney hard during the coming week, ahead of the New Hampshire Jan. 10 primary election.
During his interview on MSNBC, Romney was asked if he thought he would get a win in Iowa.
"We knew it was a stretch," said Romney, adding his campaign plan was to "carefully build our momentum in Iowa."