MANCHESTER, New Hampshire — Republican Senator John McCain, who lost the White House race to President Barack Obama in 2008, endorsed Mitt Romney on Wednesday to take on the embattled incumbent in November.

"I'm really here for one reason and one reason only, and that is to make sure that we make Mitt Romney the next president of the United States of America," the lawmaker from Arizona told cheering supporters in a high school gymnasium in New Hampshire.

Romney, who came to this winter-bound northeastern state after winning Iowa's caucus by a whisker over Christian conservative Rick Santorum, lost the Republican nomination to McCain in 2008 after a frequently bitter battle.

The former Massachusetts governor and millionaire venture capitalist's vast campaign warchest, national organization and high-profile endorsements leave him well poised to be the party's standard-bearer in this year's elections.

"We're now on track to retire a guy who's a nice guy but is in over his head. It's time for Barack Obama to go home," Romney said.

McCain remains popular in New Hampshire, where he beat Romney 37-31 percent four years ago, and his backing for Romney reflected an effort by the Republican establishment to avoid an intra-party bloodletting that could leave the eventual nominee in a weakened position to take on Obama.

The Democratic incumbent faces a difficult road to reelection weighed down by the sour US economy and stubbornly high unemployment four years after he promised his historic victory would bring "hope and change."

With the Iowa caucuses now out of the way, the New Hampshire primary is the the next voting step in the battle among Republican candidates for the right to face off against Obama on November 6.

McCain expressed "nostalgia" about visiting New Hampshire and urged the rowdy crowd to turn out an "overwhelmingly vote that will catapult this candidate to the White House."

"Our message to President Barack Obama is: "You can run but you can't hide from your record," said the decorated Vietnam War veteran, who accused Obama of "destroying our national security."