Mitt Romney is the momentum candidate. But why?

It’s official: Mitt Romney is the momentum candidate in the presidential race.

The Gallup daily tracking polling has shown him up three and five points on President Obama over the last two days. A CBS/New York Times survey released this morning showed the race tied at 46 percent. And a Pew Research Center poll pegged Obama’s lead over Romney at four points, down from a 12-point bulge last month.

Here’s the trend line of head-to-head polling conducted since the summer of 2011 (courtesy of Huffington Post):

It’s clear from that chart that Obama’s numbers peaked — at least in relation to a matchup with Romney — in mid-February and have been declining steadily ever since then. What’s less obvious is why.

Not surprisingly, we have a few pet theories.

The most obvious reason for the Romney bump seems to be that the Republican primary is now totally behind him.

Obama’s numbers vis a vis Romney were at their acme in the heat of a primary that saw the former Massachusetts governor struggle to prove his conservative bona fides and take a series of hits over whether he was underperforming given his financial and organizational advantages.

Now that Romney is the nominee, there’s increasing evidence that while many in the party would have preferred a different candidate, they are perfectly comfortable backing Romney in a head to head matchup with President Obama.

In the Pew poll, 88 percent of those who backed someone other than Romney in the GOP primary say they will support him in the general election including 78 percent who say there is no chance they will change their minds.

“Most of it is GOPers and independents who are essentially GOPers behaviorally all aligning behind Romney now that he’s the nominee,” said Chris Wilson, a Republican pollster, when asked to explain the tightening race.

Bob Honold, a GOP strategist, added that the “first trimester of the campaign is now over” and that “it took conclusion of the primary to begin bringing the vast assortment of anti-Obama voters together.”

Another reason for Romney’s rise (or Obama’s fall) is the willingness on behalf of the general public to give the former Massachusetts governor a second look now that he is the GOP nominee.

In the CNN poll, a majority (53 percent) of people said they plan to give Romney a “second look” when the primary season officially ends. Romney already appears to be benefiting somewhat from that second look; his favorable number improved by 10 points while his unfavorable rating dropped by 11 points from a mid-February CNN survey.

This second-look phenomenon is common in presidential politics. Winning a primary fight — no matter how contentious — tends to embiggen (thank you Jebediah Springfield!) the nominee. Romney might have looked like a flip-flopping, politician’s politician during the primary season but now — or, at least for the moment — he’s the victor, the man who fought for the right to face President Obama in the fall and won.

Romney is the fresh(er) face in the presidential political world right now and he’s getting coverage commensurate with that status. That of course, won’t last.

“Obama really has not yet begun to fight, so these numbers don’t reflect the view of Romney that will be shaped by a tough general election campaign,” predicted Democratic strategist Matt Bennett, a veteran of the Clinton White House.

The final reason that gets at why Romney and Obama are now running closer in polling is that the electoral dynamics over the past several years have always pointed to a narrow victory or defeat for the incumbent this fall.

The continuing struggles of the economy coupled with the overriding belief that the country is still headed off in the wrong direction virtually ensures that any credible Republican candidate — and Romney qualifies — will run within a point or two of Obama nationally.

Given that reality, Romney’s stock was slightly undervalued during the tail end of the primary season and is now bouncing back to its fair market value.

Here’s the deal: Whatever reason you believe has led to the tightening between Obama and Romney — and it’s probably a little of all three things we outlined above — the race seems likely to stay right around where this latest string of polls suggest it is. Romney may bump up a few points or Obama may take a 3-5 point lead. But, for the next seven months, the narrowness of the head to head matchup is the new political reality.