With Tuesday night’s upset by Republican Scott Brown in Massachusetts, the GOP gained more than just a 41st vote to disrupt the Obama agenda. As attention turns to the midterm elections in November, the Republican Party has strong momentum. A few months ago, even GOP leaders said that taking over the Senate was a pipe dream, and it is still not probable. But as some independents sour on the Democratic Party, the possibility for a GOP majority can no longer be dismissed out of hand. More likely, next year’s Senate will still have a Democratic majority but be much more closely balanced between Democrats and Republicans.

In fact, it is likely that the Republicans will gain at least 3 to 5 Senate seats in November. Even more startling, in the aftermath of the Massachusetts special election, Republicans would do even better IF the general election were being held today. The Crystal Ball projects that the Democratic majority in the Senate would be reduced to just 52 seats if November’s contests were somehow moved to January.

Luckily for the Democrats, the election is not today. By November the economy may be in much better shape, and some of the current controversies may appear less significant. Contests that would tip to the GOP today could easily wobble back to the Democrats (such as Missouri and Pennsylvania). That is why we still classify them as toss-ups overall.

At the same time, given Tuesday’s Bay State results, the Republican Party will search for, and possibly find, credible challengers for some Democratic senators believed to be safe until now. Imagining themselves as Scott Brown (on the victory stage, not in a Cosmo photo spread), a few “A” list Republicans might take a second look at the Senate and decide to jump in.

Among the senators who could be endangered by a new wave of Republican entries are Evan Bayh (Indiana), Kirsten Gillibrand (New York), Patty Murray (Washington), and Russ Feingold (Wisconsin).

An early example is Indiana, where Republican Congressman Mike Pence is reportedly weighing a challenge to Evan Bayh. The Hoosier State gave Obama its electoral votes in 2008 by a razor-thin margin. If Democrats could lose a Senate seat in Massachusetts, which Obama carried with 62 percent, it is theoretically possible that Indiana could be in play come November. The same is true for the other states. In politics you cannot beat somebody with nobody, but as Scott Brown proved, a nobody can become a somebody rather quickly in the right environment.

Post-Brown, the magic number for Republicans is 10 if the GOP is to take control of the Senate. (Vice President Joe Biden would cast the tie-breaking vote in favor of Democratic control should there be a 50-50 split.)

One wonders whether a 9-seat gain would be sufficient for the GOP, however. We all recall the May 2001 party switch of Vermont Senator Jim Jeffords from R to I-D. Might the Republicans lure Joe Lieberman of Connecticut, who sits in the Democratic caucus but is officially an independent? Lieberman is already persona non grata to liberal Democrats as well as a solid vote for the GOP on national security issues—and was nearly named the Republican vice presidential nominee by John McCain in 2008.

A lot can happen in a short time, as Tuesday showed anew. Democrats have plenty of chances to ward off this “Nightmare on November Street” if the economy and Obama’s approval ratings rebound over the next nine and a half months. For the moment, though, the Democrats’ nightmare is the Republican dream scenario, as our Senate rankings suggest.