Mitt Romney folded his presidential campaign two days after he pulled off a robust victory in Minnesota's precinct caucuses, one of his relatively few Super Tuesday high points.

"Nationally, it was too big a hill to climb," said Brian Sullivan, a businessman and Republican activist who ran Romney's campaign in the state. "He ran a good campaign, but he had a formidable opponent in (John) McCain."

Romney did well in caucus states such as Minnesota, also winning in North Dakota, "because in those states, voters are typically more engaged, which makes for a more level playing field," Sullivan said. "Republicans here spent more time getting to know him and viewed him as the more reliable conservative."

Sullivan said Romney was destined to have an uphill battle against McCain, starting with single-digit name recognition nationally, taking on someone who had run for president once before. "Candidates who've run in the past typically prevail," he said.

Despite the well-documented bad blood between McCain and Romney -- and, by extension, their supporters -- Sullivan said Romney backers are likely to eventually line up behind McCain.

"It'll take a period of time to get used to the idea of McCain as the nominee," he said. "But whatever disagreements we have with McCain pale beside those we have with Clinton or Obama. But McCain has to reach out and make sure he doesn't take the base for granted."

Sen. Norm Coleman, who supports McCain after initially backing former New York City Mayor Rudy Giuliani, issued a prepared statement shortly after Romney dropped out.

"His participation in this historic election gave a strong voice to conservative ideals that I believe will be carried forward by John McCain," Coleman's statement said. " Mitt Romney deserves the thanks of Republicans and Americans for his steadfast belief in the future of our nation."

Coleman also predicted that the party will be able to unite and support McCain.

"I'm very confident that come election day conservatives are going to be supporting John McCain, and Norm Coleman," he said in a conference call with reporters. "And by the way, independents are going to be supporting John McCain and Norm Coleman. And that conservative Democrats will be supporting John McCain and Norm Coleman. And at this point there is work to be done to get to that place, but I'm sure the work will be done and there will be much greater sense of unity as we walk down the road."

-- BOB VON STERNBERG and CONRAD WILSON