Rep. Tom Latham (R)
Latham had drawn credible but hardly terrifying opposition in the form of ex-state Sen. Staci Appel, and he was in solid shape for re-election, which seemed to explain his refusal on the Senate race. So it's very hard to explain Latham's decision. If you want to engage in conspiracy theories, perhaps Sen. Chuck Grassley secretly informed Latham that he won't run again in 2016, thus leading Latham to pull a Matheson. That doesn't really add up, though, since a Latham loss this cycle was very unlikely, so maybe he's just truly tired of politics.
Regardless, the GOP will now have another tough seat to defend, along with Wolf's (though Matheson's is pretty much a gimme for them). And plenty of Republicans could jump in to the primary to succeed Latham. Some possibilities from the gang at Roll Call include former state party chair Matt Strawn, state Rep. Peter Cownie, and state Sen. Jack Whitver, while Aaron Blake suggests Secretary of State Matt Schultz and West Des Moines Mayor Steve Gaer. In addition, most of the Republicans running for Senate appear to live in the 3rd and could thus drop down if so inclined. One more option: state Sen. Brad Zaun, who lost to Boswell by 4 points in the old 3rd District in 2010.
Other Democrats might also want to join Appel, now that the daunting obstacle Latham presented is gone. More importantly, the district will shoot to the top of the party's list of pickup targets, and outside spending is likely to be very high here. All in all, a very strange day, but one that could turn out net positive for Democrats.