It's an unlikely possibility, one that's been largely ignored as chances of a House takeover have dominated midterm discussion, but it's a long-shot possibility nonetheless, and Josh Kraushaar examines it in a post at Hotline OnCall:
...the GOP needs to win a net of 10 seats to retake the Senate -- a significant hurdle that has only been achieved three times in the last 60 years. (Perhaps the bar should be set at nine, with Sen. Joe Lieberman, I/D-Conn., a possibility to caucus with Republicans if they're within striking distance of a majority.)
But the reality on the ground is that nearly all of the competitive races are trending the Republicans' way, and the Republicans have put enough seats in play to have a shot at maximum impact if they run the table. The Cook Political Report now lists 11 Democratic-held Senate seats as leaning Republican or pure toss-ups. Republicans are defending seven seats in competitive races, but the main GOP-held seats at risk are Florida (where Gov. Charlie Crist could win as an Independent and become a nominal Democratic vote) and, to a lesser extent, Kentucky and Missouri. In wave elections, toss-up seats traditionally break one way, in the direction of the party with momentum.
Under perfect circumstances of Republicans winning most of the competitive races this cycle, it could happen. According to current polls, Republicans are poised to pick up seats in North Dakota, Indiana, Arkansas, and Delaware, where GOP candidates lead by double digits in polls. In New Hampshire, the Republican candidate leads by eight percentage points. Democrats are engaged in close races to defend seats in Washington, Nevada, California, Illinois, Pennsylvania, Wisconsin, and Colorado. If Republicans win all those races, it would net them 12 seats. Should Linda McMahon use her vast personal wealth to pull off an upset in Connecticut, Republicans would take over 13 Democratic-held seats.
Democrats, meanwhile, are by no means a lock to win any Republican-held seats, but they are threatening in a number of states. In Missouri, Secretary of State Robin Carnahan trails Rep. Roy Blunt by single digits; the same for Jack Conway in Kentucky, where Republican Rand Paul's campaign keeps hitting snags. In Florida, independent Charlie Crist, who, as Kraushaar notes, could vote Democratic more often than not, leads a multi-way race. In North Carolina, Elaine Marshall remains within striking distance of Sen. Richard Burr. That puts four Republican seats in realistic play for Democrats.
If Republicans win all 13 of the races above (tough to do) and Democrats capitalize on their own four opportunities (also tough), Democrats and Republicans would each control 49 votes, and the Senate majority would be decided by Crist and Lieberman.
As Kraushaar notes, wave elections can see competitive races break one way or the other, so a GOP takeover remains an outside possibility--stress outside. The lists above are both a bit optimistic, from each party's standpoint, and there are lots of variables that will play out between now and November. But the Senate will probably shift toward the GOP.