Six months ago, the 2010 Senate battlefield looked relatively bare, with a few obvious skirmishes mostly in states with GOP incumbents. Three months later, the outlook had brightened dramatically for Democrats, largely the result of a number of GOP retirements and solid Democratic recruiting on those open seats.

[IMGCAP(1)]But now, as the dog days of summer begin, the landscape has shifted again, this time improving significantly for Republicans.

Democrats no longer have the momentum they once possessed. Even more important, signs of some Democratic vulnerability have appeared, giving the National Republican Senatorial Committee opportunities to shoot at, rather than forcing it to play an entirely defensive game, as it has the past two cycles.

Fifteen months before the midterms, Democrats have major problems in two states — Illinois and Connecticut — while a third, Nevada, remains a potential headache. Republicans, on the other hand, have serious vulnerabilities in four states — Kentucky, Missouri, New Hampshire and Ohio — and potential problems in two others. But of late, even those Republican vulnerabilities look less daunting than they once did.

The announcement by Illinois Attorney General Lisa Madigan (D) that she will seek re-election rather than run for the Senate (or governor) immediately boosted Republican prospects in what remains a very difficult state for the GOP. But Rep. Mark Kirk (R-Ill.) should be a formidable candidate, while Democrats have a field that is less than intimidating.

And in Connecticut, veteran Sen. Chris Dodd (D) has aired multiple TV ads in an attempt to remind Constitution State voters what he has accomplished and what he stands for — an open acknowledgment that he has work to do to repair his image. Republicans now worry that Dodd, who just announced he will have surgery for prostate cancer, will retire rather than seek re-election, thereby damaging their prospects of winning the seat.

Democrats have two formidable candidates in Kentucky, while Republicans recently received a gift from Sen. Jim Bunning (R) when the endangered two-term incumbent announced that he would not seek a third term. That means Secretary of State Trey Grayson will likely be the GOP nominee, dramatically increasing the chances that Republicans can retain the seat.

Former New Hampshire Attorney General Kelly Ayotte (R) is moving toward a Senate bid in the Granite State’s open-seat contest, and while she is not yet a proven campaigner, insiders who know her speak effusively about her abilities and appeal. Democrats once viewed their likely nominee, Rep. Paul Hodes, as a solid favorite to win the seat, but the race now looks like a tossup, at best, for Democrats.

Meanwhile President Barack Obama’s sliding popularity is at least a troubling sign for Democrats in both Missouri and Ohio, where Republican Senate candidates may benefit from the public’s growing concerns about federal spending, possible tax hikes and bigger government.

Republicans still lack a top-tier challenger to Senate Majority Leader Harry Reid (D-Nev.), and Sen. John Ensign’s (R-Nev.) recent personal troubles certainly don’t boost Republican prospects next year. Still, as the president’s point man in the Senate, Reid simply makes himself a juicy target in the midterm elections.

Democrats have potential opportunities in North Carolina and Louisiana, but they still have work to do in both. The party has not yet recruited a serious threat to Sen. Richard Burr (R-N.C.), and while Rep. Charlie Melancon (D-La.) is widely rumored to be leaning toward a challenge to Sen. David Vitter (R), the state’s fundamentals and the midterm environment raise questions about the viability of the challenge.

Republicans have three longer-shot opportunities that shouldn’t yet be completely discounted — Arkansas, Colorado and Pennsylvania — though in each case the Democratic incumbent has a considerable advantage. Pennsylvania, in particular, is intriguing, since a truly nasty Democratic primary seems likely and the GOP nominee, former Rep. Pat Toomey, is not without appeal.

Eleven Republican and 12 Democratic Senate seats up next year now look safe. But if Rep. Mike Castle (R-Del.) decides to run for the Senate, as some GOP insiders now believe he will, another of those safe Democratic seats suddenly becomes a tossup.

Republicans would be wise not to celebrate just yet. Their diminished vulnerability is, in part, the result of Pennsylvania Sen. Arlen Specter’s leaving the GOP, which cost them a seat that they probably were going to lose next year. And with Democrats controlling 60 of the Senate’s 100 seats going into next year’s elections, any additional Republican losses would add to the party’s existing woes.

The widely expected resignation of Sen. Kay Bailey Hutchison (R-Texas) in the fall, which will lead to a special election in the first half of 2010, also creates some uncertainty. While Republicans will have a strong nominee and the NRSC will spend what it takes to hold the seat, the special election is at least a major distraction for the national GOP.

If politics is about momentum and message, then the outlook for ’10 has changed considerably over the past couple of months. Democrats still have a wealth of opportunities and some advantages, but Republicans now have momentum and an improving issue mix. For the first time this cycle, I can imagine a scenario where Democrats do not gain Senate seats in 2010.

Stuart Rothenberg is editor of the Rothenberg Political Report.