Not gonna happen. Or at least: there's no precedent for anyone jumping in this late and seriously contesting for the nomination.
Yes, I know that some candidates have declared their candidacy later than this, but Bill Clinton (for example) hardly started running at the last minute. Ryan, as far as I know, has done basically nothing in the way of campaigning so far.
I mean, really: James Pethokoukis has this late entry coming...I don't know, after the Ames straw poll?
But if Mitt Romney, Tim Pawlenty, Newt Gingrich — maybe Mitch Daniels, too — fail to catch fire, expect the pressure on Ryan to run to rise.Let me explain something. There are lulls in the presidential nomination process during every cycle, more or less built into the process, but the press on the presidential beat has to keep writing about it. And they're not going to keep writing "Romney and Pawlenty are likely to win, unless Huck or Palin or maybe Daniels becomes a full active candidate" over and over again. So we get wild speculation during the lulls. New candidates! Brokered convention scenarios! Third party candidates!
And then the next round of visible events happens, and one of the boring old candidates "wins" that round, and the press will go all goofy in the knees about how that candidate has "grown" or whatever, and everyone forgets about Paul Ryan or Donald Trump or Colin Powell or Warren Beatty or whoever else was supposedly going to enter and upset everything. Until, of course, the next lull.