By John Whitesides
WASHINGTON (Reuters) - Mitt Romney's march to a possible Republican presidential nomination just got a lot longer and harder.
Front-runner Romney left Tuesday's round of three nominating contests with another reminder of his own shortcomings, after a two-state winning streak that had placed him firmly in the driver's seat in the nomination race.
Bad losses to rival Rick Santorum in Colorado, Missouri and Minnesota raised more questions about whether conservative Republicans are ready to give their hearts to a millionaire former Massachusetts governor who once supported abortion rights and a government requirement that people have health insurance.
Romney may still be the front-runner in the race to pick the Republican candidate to challenge President Barack Obama in the November 6 U.S. election. But nothing is coming easily for him in this most volatile of Republican nominating races.
"This shows Republicans are not ready yet to just automatically pull the lever for Mitt Romney," Republican strategist Ron Bonjean said. "He still has to seal the deal."
The next big showdowns will be in Michigan and Arizona on February 28. Romney grew up in Michigan, where his father was a former governor and car executive, and Arizona could be another high-stakes showdown similar to Florida.
"He wanted to run through February and roll into Super Tuesday as the presumptive nominee, and that's just not going to happen now," Republican strategist Ford O'Connell said. "This is a wake-up call for Romney, but it's not the end-all."
Super Tuesday is March 6, with contests in 10 states.
Romney's campaign will try to paint Santorum, a former U.S. senator from Pennsylvania, as another old Washington hand who backed big government spending during his time in Congress.
"I think we'll see differences in approach that will be explored," Romney senior adviser Stuart Stevens told reporters in Denver. "I just don't think it's a time when people are looking to Washington to solve problems with Washington."
Romney, a former head of a private equity firm, has touted his business experience as the cure for an ailing economy in states like Florida and Nevada, where high unemployment and depressed housing markets made the economy a top concern.
But in Midwestern states like Iowa, Missouri and Minnesota - all won by Santorum - that has not been enough to sell a more rural and conservative electorate.
Romney also could face second-guessing about his post-Florida strategy. He demolished Newt Gingrich there last week with a wave of negative attack ads, but since has largely ignored Gingrich and Santorum to aim his criticism at President Barack Obama.
He also largely skipped campaigning in Missouri and Minnesota to focus instead on Colorado, which he won in 2008 and was expected to win on Tuesday.
"Team Romney might need to tweak its strategy. So far they've been successful in going negative on their opponents and touting his business experience," O'Connell said.
'HUNGRY FOR MORE'
"But obviously Republican primary voters are hungry for something more. A lot of folks see him as a single-issue candidate right now," he said.
Santorum has been happy to stay out of the mud-slinging battle between his two rivals and portray himself as the issue-oriented true conservative in the race.
Romney's campaign began lobbing criticism at Santorum over the past two days, hitting the former U.S. senator from Pennsylvania for backing spending bills and local spending projects known as earmarks when he served in the Senate.
Romney's campaign still seems confident its financial and organizational muscle will be enough to ultimately win the nomination, even if the conservative base is not enthralled.
Romney has far outpaced his Republican rivals in raising money, with nearly $57 million brought in by his campaign and $30 million raised by the pro-Romney "Super PAC."
Sensing he was headed for a bad night, the campaign sent reporters a memo on Tuesday outlining why he would win the nomination in August even if he stumbled on Tuesday.
"Governor Romney will be competing across the country and collecting delegates in state after state, even if other candidates pick up some wins," Romney's political director Rich Beeson said.
In an interview with conservative radio host Scott Hennen earlier this week, Romney shared his vision of how the campaign would play out, and it did not include a primary campaign that lasts all the way to the convention in August when the nominee formally will be named.
"Most likely, the party regulars will surround one of the people who they think has the best chance of beating Barack Obama, and start raising money for the general election," Romney said.
But a confident Romney campaign had expected February would be a good month, too, with many of the contests coming in states he won in 2008 - including Minnesota and Colorado.
While Romney had not campaigned in Missouri or Minnesota, the loss in Colorado, where he spent the last two days, was harder to shrug off.
"Losing Colorado is a five-alarm fire for Romney," Bonjean said.
(Additional reporting by Steve Holland in Colorado; Editing by Will Dunham)