There's a regular joke that has been employed after pretty much every single Republican debate and practically every Republican primary and caucus this election cycle: tonight's big winner was … Barack Obama.

It is perhaps the most cutting and yet appropriate critique of the GOP nomination fight. The more America sees of the prospective Republican candidates and the longer this race goes on, the more four years of Barack Obama doesn't seem like such a bad thing.

Rarely has Obama's advantage been more obvious than after the past few days – and, in particular, after Tuesday night's stunning and wholly unexpected defeat for presumptive GOP frontrunner, Mitt Romney. Romney didn't just lose to Rick Santorum – he got destroyed. In Missouri, he trailed Santorum by 30 points; in Minnesota, he finished third, behind Ron Paul, with a paltry 16% of the vote (in a state he won handily in 2008); in Colorado, another state he won in 200, with 60% of the vote, Romney took just 34% to Santorum's 40.2%.

Now, granted Romney didn't put up much of a fight in either Minnesota or Missouri – and the Missouri vote was purely a beauty contest with no actual delegates at stake. But in Colorado, he devoted significant time and resources and led in pre-election polls, but still lost. This was as bad a night as a frontrunner has had in a primary race, maybe ever.

Just one week ago, after Romney's convincing win in the Nevada caucus, the former Massachusetts governor appeared to have the political wind at his back. But in just a matter of days, his brief momentum has been completely upended and the creeping doubts about his candidacy, which he appeared to have silenced, have loudly returned.

It's not just the election results that show how quickly the tide has turned.

First, there was Friday's jobs report, which indicated that unemployment in the US has dropped to 8.3% and that the US economy may slowly but surely finding its way out of the hole it's been buried in for the past two years. This is almost certainly a more important story than the returns in Minnesota and Missouri. The entire rationale – indeed, one might say only rationale – for Romney's campaign against Obama is that he will be a better steward of the US economy. But if the economy continues tentatively to improve between now and November, neither Romney nor any other Republican will have a very compelling message to take to the electorate this fall.

Next came a Washington Post/ABC poll that showed Romney was trailing President Obama by six points in a general election match-up, and that by a two-to-one margin, the more voters learned about Romney, the less they liked him. This comes on the heels of earlier polling that indicates Romney has unfavorabilities above 50%. That isn't dangerous territory for a presidential candidate, it's fatal ground.

It's often the case that candidates are strengthened by long primary battles – certainly, that was the case for Barack Obama in 2008. But the opposite effect is happening with Romney. Facing off against a band of second-tier rivals that includes a former senator who lost his last Senate election by 18 points, a former speaker of the House who was last relevant in national politics when email was an emerging technology, and a congressman who wants to return America to the gold standard, Romney has won only three states. In fact, Santorum has now won more primary and caucus races than Romney. Anyone who told you that was going to happen five weeks ago would have been laughed out of the room.

But as the Republican party has moved further and further to the right, Romney's less-than-stellar conservative bona fides has been his Achilles' heel. While he leads his rivals in the all-important delegate count, it's fairly obvious that he has yet to capture the hearts of the GOP rank and file. Indeed, the entire tale of the Republican nomination race can be seen as an unceasing effort on the part of GOP voters to find someone, anyone, to cast their ballot for who isn't Mitt Romney.

When all is said and done, Romney is still likely to be the Republican nominee. He has the most money, the most establishment support, the strongest ground game and is probably the best-equipped of all the candidates for a long slog toward the nomination. But if the last few days have shown us anything, it is that Romney is a seriously flawed candidate who has rather dramatically failed to seal the deal with conservative GOP voters.

And every day longer this race goes on, the more damage will be done to Romney's hope of winning the White House. In short, last night was yet another great night for Barack Obama.