Key Senate races are tightening as candidates on both sides make unexpected gains, suggesting that the final days in the battle for control of the chamber could be as volatile as any in recent memory.
Democrats who were all but written off, including Rep. Joe Sestak in Pennsylvania and Sen. Michael Bennet in Colorado, have revived and are pulling even with their Republican opponents in some polls.
The trend could mean that party loyalists, who may have been undecided at the campaign's outset, are coming home to their candidates as election day nears. The seesaw poll numbers also reflect activists' counter-reaction whenever the other side starts to build a lead.
"The further it starts to move in one direction, the more it energizes the opposite party," said Lara Brown, a political scientist at Villanova University near Philadelphia.
If Republicans win the Senate, they could more easily control the agenda by sending President Barack Obama legislation he would have to sign or veto. Democratic control of the Senate would provide a tie-breaker of sorts between a Democratic White House and a House that Republicans are expected to win.
Republicans need to capture 10 seats now held by Democrats to win a 51-49 Senate majority. Eleven Democratic seats are in play. Three are likely GOP pickups and the rest are too close to call. Democrats are within striking distance of one Republican-held seat, in Kentucky.
The most notable campaign shift has been in Pennsylvania. Polls there suggest Sestak, who had trailed in many surveys, is taking a small lead over Republican Pat Toomey. More polls will surely will be conducted in the 10 days remaining before the vote, but the numbers show the race closer than many analysts believed.
Democrats say they have been helped by the nomination of tea-party backed Republican Christine O'Donnell for a Senate seat in neighboring Delaware. The Sestak campaign has sought to associate Toomey with O'Donnell, noting that both are supported by tea-party activists.