Back during the 2008 presidential campaign, we coined a term called tipping-point states. A tipping-point state was one that might potentially put Barack Obama (or John McCain) over the top to his 270th electoral vote, thereby winning him the presidency. States like Colorado, Virginia, Florida and Ohio were ranked highly by this metric.

The same calculation can be applied to this year’s Senate election. Which states are most likely to make the difference between Republicans controlling the Senate by exactly one seat, and falling exactly one seat short of doing so?

Unless there is significant movement in another state that defies the current consensus of polls, the answers are Washington, West Virginia, and California.

Take California, for example. Our program runs 100,000 simulations each night. In 14,102 of the simulations that we ran tonight, Republicans finished with exactly 50 seats; in 8,682 of them, they finished with exactly 51.

Republicans won California in just 4 percent of the simulations when they won 50 seats, but in 21 percent of the simulations when they won 51 seats. The difference between these two figures (21 percent and 4 percent) is 17 percent. In essence, this number — 17 percent — reflects the percentage of the time that California made the difference between Republicans controlling the Senate and not.

Ranking higher by this metric was West Virginia. Republicans won the Mountain State in 62 percent of the simulations when they won 50 seats, but 93 percent of the time that they won 51 seats. The difference — 31 percent — is how often West Virginia was the tipping point state.

But the most important state of all was Washington. Republicans won Washington 88 percent of the time that they won the Senate by a single seat, but only 41 percent of the time that they fell one seat short. The difference there is 47 percent. Washington makes the difference between success and failure almost half the time. Any polling in the state, therefore, has the potential to make a big difference in their takeover chances.

Indeed, Republicans got a good poll in Washington tonight, although it comes with some qualifications. The survey, from Rasmussen Reports, gave Dino Rossi a 1-point lead over Patty Murray, the Democrat. Rasmussen’s previous survey of the state had given Ms. Murray a 3-point lead.

Now for the qualifications: Rasmussen has had a rather different impression of the state than other pollsters. Since Labor Day, Rasmussen and its subsidiary Pulse Opinion Research has polled the state seven times; four of their polls put Mr. Rossi ahead. By contrast, there have been nine polls of the state by companies other that Rasmussen. All nine of them gave Ms. Murray a lead, although usually by small margins.

Washington has given Rasmussen problems in the past; they’ve missed high on the Republican’s standing by an average of 4-5 points since 2000. I’ve theorized that this could be because they don’t bother to adapt their likely voter model for the fact that the state votes almost entirely by mail, which could render some of the traditional questions that pollsters use to determine the likelihood of voting (e.g. “do you know where your polling place is?) nonsensical. Another complaint is that Rasmussen carelessly allows their respondents to pick “some other candidate” in their polls of Washington (as 3 percent of their respondents did) — even though state law requires that only two candidates appear on the general election ballot.

But here’s the problem for Democrats. If it were just a matter of Rasmussen saying one thing and everyone else disagreeing, there would be decent reasons to be dismissive of their poll. No pollster other than Rasmussen, however, has released a survey in Washington in more than a week, at which time there had been some hints that the race was tightening. There’s a plausible hypothesis that this tightening was real; if so, this Rasmussen poll could confirm it.

Then again, even if the race has in fact tightened, it might not be enough to help Mr. Rossi going into the election, as Ms. Murray had held leads in the high single digits (and one lead of 15 points) in a few other recent surveys.

Weighing this evidence, our model has Mr. Rossi’s chances improving to 20 percent from 15 percent. As a result, the Republicans’ chances of taking over the Senate are slightly improved — from 12 percent to 13 percent (or 14 percent if Charlie Crist both comes back in the Florida Senate race and decides to conference with the Republicans).

Speaking of Mr. Crist, two recent surveys suggest that he’s made up some of his difference with Marco Rubio as support for Democrat Kendrick Meek — whom some high-ranking Democrats have been trying to urge out of the race — has dropped. Mr. Crist remains about 12 percent behind Mr. Rubio in our polling average, which would ordinarily be too large a difference to make up at this late stage. But three-candidate races are subject to unusual dynamics — for instance, Mr. Crist could continue to gain votes from Mr. Meek if Democrats conclude that Mr. Meek is not viable — and a comeback remains at least theoretically possible. The model puts Mr. Crist’s chances at about 10 percent.

The rest of the Senate polling was of less consequence today. A new poll in Wisconsin, from Wood Communications Group, shows Russ Feingold having narrowed his deficit with Ron Johnson to 4 points from 8. This is something of a good news/bad news situation for Mr. Feingold, however. What Mr. Feingold really needs at this point are surveys showing him within a point or two of Mr. Johnson, at which point same-day registration — Wisconsin is among the only states where voters can register on Election Day — could push him over the top.

Still, the model has had Mr. Feingold’s prospects improving slightly in recent days, and now has his chances of winning at 13 percent. That’s slightly better, for instance, than Joe Sestak of Pennsylvania, whom the model has at 12 percent.