Mitt Romney has picked up an unexpected and potentially game-changing endorsement from U.S. Rep. Michael Grimm of New York.

Grimm is far from a nationally known figure, so his endorsement doesn't carry the same weight as, say, the one Romney received from New Jersey Gov. Chris Christie earlier on Tuesday. But what is significant is that Grimm -- who represents Staten Island and part of Brooklyn -- is conservative and a self-identified Tea Party supporter, which is exactly the demographic that Romney has had trouble wooing.

The significance of Grimm's endorsement is the message that came with it: that Republicans should nominate a candidate who can beat President Obama, even if that candidate is not the most conservative of the bunch.

This is About Winning in 2012

Republicans need to understand this is about winning in 2012, Grimm told ABC News after announcing his endorsement. They may feel Gov. Romney is not conservative enough for them. I would counter-argue that he's certainly more conservative than President Obama. Sometimes you don't get everything you want, but if we want to save our country ... what Republicans can't do is turn this into a purity contest.

That is a good description of what the race for the Republican nomination has become. Support for Romney has remained relatively steady throughout the process, and he has almost never slipped below second place in the polls. But more often than not, he has been eclipsed by more conservative candidates who get lots of media attention, rise quickly in the polls and then fade.

It happened with Donald Trump, with Michele Bachmann and with Rick Perry. Now it is happening with Herman Cain, although it is too soon to say whether Cain's lead will last or whether he will end up being just another blip in a series of fleeting front-runners.

Grimm is no moderate, but he said the trend toward more conservative candidates could cost Republicans the election.

Says Extreme Conservativism Could Cost GOP 2012 Election

I understand voters on the far right that are looking at Michele Bachmann on principal, but she can't win, he told ABC. Why waste the time and energy and money, which is only diverting our energies from beating President Obama? It's self-defeating, splintering, and we're not keeping our eye on the ball. ... You should back whoever is going to run, but don't back the candidate you know can't win because they are more pure.

It was a not-so-subtle jab at Bachmann, who has urged conservatives not to settle for anything less than a strong constitutional conservative. She said at a debate last month that because Obama's approval ratings are so low, a very conservative candidate would be electable. But Grimm disagrees.

He argued that if Romney were the nominee, even historically Democratic states like New York could be up for grabs.

New York has not voted for a Republican candidate since Ronald Reagan in 1984. Obama won the state in 2008 by 25 percentage points, claiming 62 percent of the vote to John McCain's 37 percent, and in each of the three elections before that, the Democratic candidate received 58 percent of the vote.

But, Grimm said, Gov. Romney can do it and do it in New York because of this election cycle, because President Obama has failed to lead in so many respects and a lot of people are disgruntled. We can actually flip this state.

If we splinter, then President Obama wins, he added. We need to get down to the basic question: do we or do we not want to beat President Obama in 2012?