Senate update: What about Joementum?

The most recent Senate forecast update from Nate Silver’s Five Thirty Eight, published last Thursday, shows the GOP with a 22 percent chance of winning at least 51 seats in the U.S. Senate. (That percentage would be in the mid-30s — i.e., probably a bit north of 1-in-3 odds — if the Republicans in Delaware hadn’t committed an act of political nihilism by giving away a Senate seat in the name of ideological purity last month. But I digress.)

The GOP appears to be in solid position for pickups in North Dakota, Arkansas, Indiana (those first three are virtual certainties), Pennsylvania (approaching near-certainty territory), Wisconsin, and my home state of Colorado. (Hello, Senator Buck!) It also appears very likely that the GOP will hold all of the seats it’s defending, including the once-competitive states of Florida, Kentucky, New Hampshire and Missouri.

On the flip side, although Connecticut (Blumenthal vs. McMahon) and the New York special election (Gillibrand vs. DioGuardi) remain marginally in play, they’re very unlikely to be decisive in the fight for the majority. If the GOP is winning states like CT and NY, we’re looking at not just a “wave election,” but a tsunami, and they’ll undoubtedly already be at 51+ from victories elsewhere. As for Delaware? That would take a mega-tsunami. We’re talking a La Palma-level event to get Christine “I’m Not A Witch” O’Donnell elected. Maryland (Mikulski) and Hawaii (Inouye) might fall before Coons loses in DE, if 538’s model is correct. Anyway, for purposes of this discussion, all of those states can be considered Democratic holds, since in the unlikely event they flip, they won’t matter — they’ll be “extras” on top of an already-achieved majority.

That leaves five states that will most likely determine the balance of power in the Senate: Nevada, Illinois, West Virginia, Washington and California. At present, the GOP candidates in NV and IL look like very slight favorites, while the Dems have a slight edge in WV, a more substantial edge in WA, and a significant and growing edge in CA. If California continues to slip out of the Republicans’ grasp, it may well be that — as Nate Silver tweeted on Monday — the GOP needs to sweep NV, IL, WV and WA in order to take back the Senate majority.

Unless….

A third path might involve Republicans winning nine rather than 10 races, which has a 13 percent chance of happening. They then might be able to persuade either Joseph I. Lieberman or Ben Nelson to conference with them. For forecasting purposes, we do not account for this contingency, since there is no robust way to estimate the likelihood of one of these senators defecting.

No “robust” way, perhaps, but I thought it might be fun to take a stab at it. As Silver says, there’s a 13 percent chance of the GOP winning 9 races, and it looks like roughly a 10 percent chance of the GOP winning 8 races. Suppose, for the sake of argument, that if the Senate is 50-50, there’s a 25 percent chance either Lieberman or Nelson switches parties. Suppose further that, if the Dems have a 51-49 Senate majority, there’s a 5 percent chance they both switch. Together, those two scenarios would add almost 4 percent to the GOP’s overall 22 percent shot at a majority. Obviously, I’m just making up the 25% and 5% numbers, so if you think those are too high, you’d need to adjust the 4 percent downward, and vice versa. But generally, I think the Lieberman/Nelson scenario adds somewhere between 2 and 5 percent to the GOP’s chances.

Anyway, this raises an interesting question for the We Don’t Need No Stinking Castle crowd, the folks in the conservative blogosphere and twittersphere who argued that O’Donnell’s primary win was a good thing — even though it might well cost the Republicans the Senate — because it’s better to be in the minority without an extra RINO than in the majority with an extra RINO. How would those folks feel if the GOP gets a majority on the basis of a 51st vote coming from either Joe Lieberman or Ben Nelson?!? Those guys are more liberal than Castle, yet somehow, if push came to shove, I expect either would be welcomed with open arms, joining Scott Brown and the Maine-iacs (which, incidentally, would be a pretty good band name) in the Senate RINO Caucus. Food for thought.