“By defeating Mitt Romney in the Missouri primary on Tuesday, Rick Santorum nursed the niggling perception that the frontrunner for the Republican nomination has yet to close the deal with conservative voters…

“‘Tonight’s victory should put to bed the idea that the Republican nomination for Mitt Romney is inevitable,’ said Stuart Roy, an advisor to the super-PAC backing Santorum that ran television ads on his behalf in Missouri.”

“At this point in the presidential race, it’s about expectations and momentum. And the perceptions of both have changed post-Missouri at least somewhat.

“By winning in Missouri, Santorum proved three things:

“1) That Romney can lose

“2) That he can beat Romney head-to-head under the right set of circumstances

“3) That this race isn’t yet over

“Santorum is not a front-runner yet; that’s still clearly Romney. But the former Pennsylvania senator did do something to call into question Romney’s coronation.”

“On Saturday, a lot of people were wondering how much longer Rick Santorum would stay in the Republican presidential race. Tonight, he has won Missouri and likely Minnesota and he’s reemerged as the leading conservative alternative to Mitt Romney. Momentum, conventional wisdom holds, is now on his side. But if there’s one thing we’ve learned so far in this volatile 2012 race, it’s that momentum isn’t as powerful a force as it used to be…

“After Santorum won Iowa, he had underwelming performances in successive contests in New Hampshire, South Carolina, Florida and Nevada. By contrast, Gingrich finished fourth in both Iowa and New Hamphsire and was dead in the water heading into South Carolina. But he came back to trounce Romney there, who had been coming off his huge win in New Hampshire. Despite being blown out in South Carolina, Romney bounced back to run away with the race in Florida and Nevada, and some people expected him to run the table in February. Yet here comes Santorum again.”

“As the caucuses and primaries pick up pace, with Colorado and Minnesota taking center stage today, many are wondering whether the initial enthusiasm surrounding the Republican race is waning, given the remarkably low turnout in the Nevada caucuses, which came and went with little fanfare…

“In both Florida and Nevada, turnout dropped sharply from 2008. Florida saw nearly 280,000 fewer voters in its primary last week, while more than 11,000 fewer voters turned out to vote in Nevada on Saturday compared to four years ago. Mitt Romney won both states by a landslide.

“Some observers say the declining interest is a reflection of the lack of enthusiasm for the former Massachusetts governor.”

“What it did not mention was that caucus turnout dropped precipitously from the 2008 contest, and that Romney did not perform as well on Saturday as he did then.

“Ryan Erwin, Romney’s consultant in Nevada, put it this way, ‘This year, the excitement is different: This isn’t about going to Disneyworld; it’s about paying your mortgage.’

“Maybe so, but in Florida and New Hampshire, the former Massachusetts governor’s favorability ratings have declined as negative campaigning in the GOP primary has raged.”

“‘I feel great that Minnesota is going to change the direction of this race tonight,’ Santorum told supporters at a Tuesday afternoon rally in Blaine, Minn. ‘Governor Romney’s been able to bully his way through this primary . . . but he’s not inspiring. He’s not painting the vision.'”

Update: “Conservatism is alive and well.”

Visit msnbc.com for breaking news, world news, and news about the economy