How to explain this budding “bromance,” as first lady Michelle Obama puts it, between her husband and Bill Clinton?

The 44th president and the 42nd president are indeed having a whirlwind affair in the closing days of the campaign. President Obama takes Clinton on three campaign stops next week, stopping in Orlando, Youngstown, Ohio, and Northern Virginia to kick off the last full week of the race.

An ad Clinton cut, defending Obama against all “the stuff some folks are saying,” is set to run in those states and in Nevada, Colorado and Iowa. Since their embrace on the stage of the Democratic convention last month after the former president’s rousing speech, Clinton has been stumping and fundraising across the land for the man he once accused of peddling a “fairy tale” to the American public.

Sen. John McCain floated a theory last week about the bromance. The Arizona Republican, in a conference call for the Romney campaign Tuesday, told reporters that there are “some that think this may have a lot to do with 2016 and the president’s wife, Secretary of State Hillary Clinton. Of course, I would never suspicion such a thing, but there are some real jerks around who think that might be the case.”

I am one of those jerks. I don’t subscribe to the prevailing theory that Clinton’s efforts to boost Obama are about solidifying his own legacy. Clinton is a shrewd strategist, and he surely must have grasped that it is highly unlikely that Hillary Clinton, or any other Democrat, will win in 2016 if Obama does not win in 2012.

This is, as Obama likes to say in another context, just a matter of math. By just about all accounts, the U.S. economy will be dramatically stronger in 2016 than it is today, and an economy growing at a healthy clip almost always means victory for the incumbent party — whichever party that is.

On Friday, the Commerce Department reported that gross domestic product in the third quarter grew at a 2 percent annual rate, up from 1.3 percent in the second quarter. That’s still sluggish, but in four years, the economy should be well into its long-delayed expansion. The Office of Management and Budget projects the economy will be growing in 2016 at an annual rate of 4 percent after inflation (with unemployment at 6 percent and falling.) The Congressional Budget Office has forecast real GDP growth of 4 percent. The private-sector Blue Chip Consensus forecast has real GDP growing at 2.9 percent.

Mitt Romney’s economists are evidently making similar assumptions. They argue that under the GOP challenger’s proposals, the recovery “will align with the average gains of similar past recoveries,” resulting in 12 million jobs created by the end of 2016. As The Post’s Glenn Kessler has pointed out, private forecasters expect the economy to add that number of jobs with or without Romney’s policies.

Of course, any number of unknowns — wars abroad or debt crises at home — could make 2016 look very different. But from here it appears that if the Democrats can squeak through this election, a hostile one for the incumbent party, they will have a much easier time of it in 2016. Alternatively, if Republicans win now, their policies will get credit for the economic expansion, and they will cruise to victory in 2016.

Hillary Clinton continues to deny interest in another run, but she is already inching away from a similar vow that she would not serve as secretary of state in a second Obama term. “A lot of people have talked to me about staying,” she told the Wall Street Journal in a recent interview.

This could give her time to repair any damage to her reputation caused by security lapses that led to the death of the U.S. ambassador to Libya and three other Americans. Her longtime aide in charge of protecting her image, Philippe Reines, has engaged in a series of high-profile squabbles with media outlets recently, which is more suggestive of an aspiring candidate than of a retiring diplomat.

Her campaign theme is already being prepared by her husband, who is establishing that the expansion of the next four years will be attributable to Clintonian methods. “We created 22 million new jobs and turned deficits into surpluses,” Clinton says in his new ad for Obama. “President Obama’s got it right. We should invest in the middle class, education and innovation, and pay down our debt with spending restraint and asking the wealthy to pay a little more. Sound familiar?”