The most important polling number of the day? I’ll vote for Tammy Baldwin leading Tommy Thompson, 47 percent to 43 percent, in polling from the very well-regarded Marquette Law School team.

Not only is that bigger than the 3.1 percent lead Baldwin had previously held in the HuffPollster average, but Marquette’s previous survey had supplied Thompson with his only non-Rassmussen lead since Baldwin moved into the polling lead in September.

Why is this a big deal? First of all, because it means that there’s a very good chance that Tammy Baldwin is going to be a United States Senator. There’s some symbolic importance for her as an open lesbian getting elected to the Senate. But perhaps a bigger deal is that there’s every expectation that she will be an active, hard-working liberal in a chamber which often seems to have few of those. Individual Senators can make a great deal of difference. Influence depends to a large extent on the Senator’s abilities, not (as in the House) their committee or leadership position.

But party majority (and the size of the majority) matter quite a bit as well, and that’s the other reason this poll, and the implication that Baldwin is in pretty good shape, matter quite a bit. Wisconsin has been ranked as the Democrats’ 51st Senate seat: that is, there were 50 seats with either a Democrat incumbent who isn’t running or a Democrat currently winning by more than Baldwin’s 3.1 percent. Subjectively, I’ve always thought she was more likely to fade than some of the other “lean Democratic” leaders, such as Elizabeth Warren in Massachusetts or Sherrod Brown in Ohio. But that doesn’t seem to be the case (perhaps because Tommy Thompson has proven an awful candidate for this cycle).

In other words, this poll, along with the other evidence of the last several days, make it highly unlikely that Republicans can win back the Senate. And don’t forget: In addition to the 51 Senators Dems will have if they hold their current clear leads, there are another three tossup states (Virginia, Montana, and Indiana) where Democrats currently hold apparent polling leads, and four others (North Dakota, Arizona, Nevada, and Nebraska) where Republicans have small leads. It’s now probably more likely than not that Democrats will at least retain 53 seats in the new Senate, and they could easily have more.

In all, it’s shaping up to be an excellent cycle for Democrats in the Senate; we’re close to the point where the only question is how excellent it will be.