Romney traded blows with McCain for several weeks earlier this year before dropping out of the race and conceding defeat after losing crucial primary contests on Super Tuesday on Feb. 5.

With past battles behind them, Romney joined the Arizona senator in Salt Lake City, Utah, at a fund-raising event on Thursday, and then flew with him to Denver, Colorado.

“It’s a lot of fun again. It’s nice not to feel any pressure at all, I don’t have to worry about goofing up,” he told reporters on the flight over the Rocky Mountains.

“I can just stand behind the nominee and do my very best to support his campaign.”

Romney, a former governor of Massachusetts, and Gov. Jon Huntsman of Utah stood beside McCain at the campaign stop in Salt Lake City in a show of party unity.

McCain said he hoped Romney would join him on the campaign trail in the weeks ahead as he sought to energize the party in the run up to the election in November.

He will face Democrats Sen. Barack Obama of Illinois or Sen. Hillary Clinton of New York.

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- Photo credit: Reuters/Rick Wilking (McCain (L) listens to Romney at a news conference in Denver on March 27, 2008)