Makes sense, really.

The pro-Romney groups American Crossroads and Americans for Prosperity are pouring nearly $13 million into advertising in key states, indicating they remain eager to lend considerable financial muscle to Romney in states viewed as truly competitive.

There are no presidential campaign ads of any kind airing in Pennsylvania and Michigan, according to information provided by media trackers to the Associated Press…

“You don’t spend money if you know you’re going to win or you know you’re going to lose, and Obama hasn’t spent five cents” in Michigan, Robinson said…

Romney’s forces could be waiting to see how he and the president emerge from their national conventions, Robinson said. The Democratic National Convention in Charlotte, N.C., ends today. The Republicans met last week in Tampa, Fla., where deputy campaign director Katie Packer Gage told the Michigan delegation that Romney “will never pull out of Michigan.”

There’s no reason to spend in Pennsylvania. Realistically, the only scenario in which Romney takes it is if there’s a huge red wave that not only carries Mitt to victory in the battleground states he needs for 270 but even in a few that he doesn’t need, like PA. If the polls there start to tighten down the stretch, then they’ll almost certainly be tightening in purpler, more important states like Virginia and Colorado too. If you had $5 million to spend on ads, where would you rather roll the dice? On Virginia, where O might be up by a point in October, or in Pennsylvania, where he’s apt to be up by five points or more?

As for Michigan, O leads there right now by 2.4 points in the RCP average, although most individual polls over the past six weeks have it closer to five or six points. I think this is going to be one of Romney’s “Hail Mary” states if he falls behind in October and suddenly needs to make a desperate play for electoral votes as other battlegrounds start to slip away. To refresh your memory, here’s how centrist Dem William Galston frames the math:

The Obama 2 are Wisconsin (10 EVs), which Obama won by 14 points in 2008, and Michigan (16 EVs), where he prevailed by more than 16. Polling this year has long indicated much closer races in these states, and the selection of Paul Ryan seems to have contributed to the statistical ties shown in the most recent surveys. Winning either of these states would be a game-changer, broadening Romney’s options for reaching 270 electoral votes and narrowing Obama’s.

But for simplicity’s sake, assume that each candidate does what he must, with Romney taking Florida, North Carolina, and Ohio while Obama turns back the Republican assault in the upper Midwest. If so, Obama would have 247 electoral votes; Romney, 253. And five states would be left to decide the contest: Virginia (13), New Hampshire (4), Iowa (6), Colorado (9), and Nevada (6). Obama won all five in 2008, four by margins exceeding his national margin, the fifth (Virginia) with slightly less.

If Michigan stays reasonably close — which isn’t impossible, given the Romney family’s ties there — while most of those five true toss-ups start to drift away, then Team Mitt, the RNC, and conservative Super PACs may go all-in on Michigan as a last-ditch EV jackpot. But clearly, they’re not going to do that now. Better to carpet-bomb the toss-ups and try to move the needles there than throw for the end zone in a state won by Obama four years ago by, er, 16+ points.

And speaking of carpet-bombing, it’s time to duck and cover, my friends. Here it comes. Exit quotation:

Senior Romney-Ryan campaign officials tell Fox News the campaign will launch an enormous media offensive on Friday, the day after President Obama accepts the Democratic Party’s nomination for a second term. The push will include ad buys in several states that will cost tens of millions of dollars…

Romney-Ryan officials did not repudiate such talk; indeed, one official, in speaking to Fox News, likened the offensive that will begin Thursday to the “daisy cutter” bombs used in the Iraq war.