Rick Santorum, in the biggest surprise of the night, took Colorado, a state Mitt Romney had won with 60 percent in 2008. That gave him a sweep of all three contests last night (although Missouri’s result will not impact the actual delegate selection). It was by any measure a hugely impressive evening for a candidate who had not won since Iowa.

Santorum was pugnacious and feisty in his victory speech in Missouri earlier in the evening. Mitt Romney, speaking before the Colorado contest was decided, was calm, gracious and methodical. It was in a real sense a vivid portrait of the difference, as much temperamental as substantive, between the two top contenders.

Romney didn’t spend much time on Santorum, other than to jab generally at Washington D.C. insiders: “Washington cannot reform itself and Washington will never be reformed by those who have been compromised by the culture of Washington. This is a clear choice. I am the only person in this race — Republican or Democrat — who has never served a day in Washington. In the world I come from, leadership is starting a business, not trying to get a bill out of a subcommittee.”

Romney’s target was President Obama. He adopted a new refrain: holding Obama to account under the criteria he set for himself at the Democratic National Convention in 2008:

He said the “Democrats have a very different measure of what constitutes progress.”

Under his definition, progress would be measured by “how many people can find a job that pays a mortgage.”

More Americans have lost their jobs during President Obama’s term than during any other in modern history. And more Americans have lost their homes during President Obama’s term than during any other in modern history. Under his own definition, President Obama has failed. We will succeed!

In that same speech in Denver, candidate Obama said progress would be determined by “whether the average American family saw its income go up … instead of down.” During the last four years, the median income has fallen by around 10%. Under his own definition, President Obama has failed. We will succeed!

Candidate Obama went on to say that we could measure progress by “whether someone with a good idea can take a risk and start a business.” Have you seen what’s happened to small business? Last year, under President Obama, there were almost 100,000 fewer new business start-ups than there were three years before. By his own definition, President Obama has failed. We will succeed!

Candidate Obama said we could see progress in “an economy that honors the dignity of work.” Under President Obama, the average duration of unemployment has more than doubled and 14 million more people are on food stamps. By his own definition, President Obama has failed. We will succeed!

He then introduced a more populist tone, recounting his father’s story:

My father never graduated from college. He apprenticed, as a lath and plaster carpenter, and he was darn good at it. He learned how to put a handful of nails in his mouth and spit them out, point forward. On his honeymoon, he and Mom drove across the country. Dad sold aluminum paint along the way, to pay for gas and hotels.

There were a lot reasons my father could have given up or set his sights lower. But Dad always believed in America; and in that America, a lath and plaster man could work his way up to running a little car company called American Motors and end up Governor of a state where he had once sold aluminum paint.

For my Dad, America was the land of opportunity, where the circumstances of birth are no barrier to achieving ones dreams. In Dad’s America, small business and entrepreneurs were encouraged, and respected.

It was a sign Romney knows he’ll have to make headway with blue-collar voters and to connect on a more emotional level.

The question for Romney now is whether he sticks to his game plan — rolling over challengers in ads and debates, presenting himself as the most electable and emphasizing his business background — or whether he changes pace, stressing conservative policies and trying to allay the concerns of the base that he will not govern from the right. Santorum, on the other hand, will be encouraged to do more of the same: Hit Romney from the right and lay out his populist economic agenda.

The good news for Romney is that the two states at the end of the month, Arizona and Michigan, are strong ones for him. The bad news is that once again he’ll need to turn back to the primary race and engage in hand-to-hand combat. This time his challenger is more capable and less flawed.

Moreover, Santorum will enjoy, at least for a short time, a new tidal wave of positive media coverage, both from mainstream outlets and conservative ones. The former is delighted to prolong the primary excitement and see the bloodletting continue. The conservative media, a thorn in Romney’s side from the get go, is now making up for lost time and cheering Santorum on, delighted to have found a viable standard bearer for the right.

The good news for the GOP is the race is now essentially between two credible, intelligent and experienced candidates. Each will improve as time goes on. The winner will hopefully have learned something from the loser. And, with a competitive race, perhaps the GOP turnout will even tick up, building some enthusiasm for November.

Which candidate can capi­tal­ize on past successes and churn out victories in Maine, Arizona, Michigan and then on Super Tuesday? It is no more clear today than it was following the South Carolina primary.. So far, one victory has not ensured the next contest will go the winner’s way. Really, if Santorum could win Colorado just days after he finished last in Nevada (a state Romney won by 20 points) no prediction is reliable.

A final note: It was a dismal night for Newt Gingrich. If his Romney-hatred is deep will he get out and endorse Santorum? Ironically, Romney probably hopes his nemesis sticks around, taking up 15 percent or so of the not-Romney vote.