Two weeks ago, Rick Santorum led Wisconsin, in a couple of polls by double digits. Then the ads started. Mitt Romney outspent Santorum massively—I saw estimates ranging from 4-to-1 to 10-to-1—and eventually the numbers flipped. In all of these important states—Michigan, Ohio, Illinois, and now Wisconsin—Romney has eked out relatively narrow victories over a clearly unelectable opponent for one reason: money.

The most interesting story to emerge out of Wisconsin, though, is the way Romney’s win there will build speculation about him choosing Rep. Paul Ryan as his running mate. Ryan endorsed Romney, campaigned with him, handed out sandwiches with him, and introduced him Tuesday night. All this on the same day that President Obama lit into Romney by name and the Ryan budget as extreme “social Darwinism,” signaling that Romney’s support for Ryan’s budget (which features a $265,000 tax cut for millionaires and steep domestic budget cuts) will be a centerpiece of attacks come the fall.

No one made Romney tie himself to Ryan that tightly. And it’s not as if the guy is some legend in Wisconsin. He represents just a single congressional district, and in fact he is a divisive figure who usually has close races even within that district. But Ryan is a hero to the state’s right wing, and that’s why Romney did what he did. And it worked—exit polls showed he won or nearly won groups (middle-income voters, evangelicals) where Santorum swamped him in earlier states. But of course, only one side was voting last night. It’s hard to imagine Romney wanting so direct an association with the GOP’s leading lightning rod, but then again, it was once hard to imagine Romney doing a lot of the things he’s done.