WASHINGTON — Republicans held key seats in Indiana, North Carolina and Wisconsin on Tuesday and were leading hard-fought races in Missouri, and New Hampshire as Democrats' hopes of seizing control of the the Senate faded.

Democrats need a net gain of five seats to win a majority, or four seats to split the Senate 50-50. In the event of an evenly divided Senate, the party that wins the White House would effectively win the Senate majority since the vice president acts as the tie-breaking vote.

Democrats picked up at least one seat. Democratic Rep. Tammy Duckworth ousted Republican Sen. Mark Kirk in Illinois. Duckworth, who was first elected to the House in 2012, lost both of her legs in combat in Iraq; Kirk suffered a stroke during his only term in the Senate. Democrats also were leading in Pennsylvania, with former state environmental chief Katie McGinty ahead of Republican Sen. Pat Toomey.

In Indiana, Republican Rep. Todd Young beat two-term former senator Evan Bayh for an open seat created by the retirement of GOP Sen. Dan Coats. Bayh was a late entrant to the race and had an early lead in polls. And in North Carolina, GOP Sen. Richard Burr, chairman of the Senate Intelligence Committee, defeated Democratic former state representative Deborah Ross.

But the most unexpected victory for Republicans was in Wisconsin, where GOP Sen. Ron Johnson won a rematch against Democratic former Sen. Russ Feingold, who had led Johnson by double digits in polls just a few weeks ago.

The party that controls the Senate will help determine whether the new president can push his or her agenda through Congress for the next two years. If the early trend holds, the Republicans would maintain control, which would give them power over both chambers of Congress for another two years.

Democrats believed they could wrench the majority away from Republicans because the GOP had so many more seats to defend this year. There were 24 Republican-held seats on state ballots Tuesday and only 10 Democratic-held seats.

The only seat that Democrats were in danger of losing was the one currently held by Senate Minority Leader Harry Reid of Nevada. Republican Rep. Joe Heck was leading Democratic former attorney general Catherine Cortez Masto in early returns.