Facing a tough race against GOP Rep. Denny Rehberg, Sen. Jon Tester of Montana could well become the only Democratic senator to lose reelection – a sign of the Democrats’ improved fortunes since 2010. Polls show the race deadlocked, but with the issue matrix favoring the challenger. In the final weeks, Rehberg has aggressively hit Tester over his support for Obama’s environmental regulations in an energy-producing state. That was an issue that created major problems for Democratic Senate candidates two years ago, and could be the difference in a close race this time.

The Romney bounce has also solidified the standing of two Republican candidates who looked to be in tough shape several weeks ago: state Treasurer Richard Mourdock in Indiana and Rep. Jeff Flake in Arizona. Romney even cut an ad for Mourdock, his only Senate ad this cycle, a sign of improving fortunes for both candidates.

On the flip side, Democrats are showing momentum in several solidly Democratic states that Republicans were recently optimistic about: Massachusetts, Maine, and Connecticut. Sen. Scott Brown of Massachusetts is now consistently trailing Warren in polls, coming up just short of winning over the 20 percent of Democrats he needs during a presidential election year. Within Republican circles, there’s an increasing sense of resignation that Brown did everything he needed to do, but is having trouble overcoming the overwhelming Democratic nature of the state in a presidential year…

The Senate majority could very well be decided by who wins the presidential election, but not because of any notable coattail effect. Rather, the battle for the Senate is so close, it could come down to the party affiliation of the next tie-breaking vice president.