Republicans will retain their majority in the US House of Representatives, CNN projects.

Democrats have now picked off one of the five seats they need to wrest control of the Senate away from Republicans, but both parties are anxiously watching returns in six key states that will decide the chamber’s balance of power.

Democrat Tammy Duckworth will defeat Republican Sen. Mark Kirk in Illinois, according to a CNN projection.

It’s one of the five seats the party will need to flip the GOP’s 54-46 majority.

Republicans, meanwhile, held onto seats in two presidential swing states: Florida and Ohio.

In Florida, Sen. Marco Rubio — whose late decision to seek re-election after his failed presidential bid was a boon to the GOP’s chances of keeping the Senate — will best Democratic Rep. Patrick Murphy, CNN projects. Republican Sen. Rob Portman will also win re-election in Ohio, CNN projects. He ran what’s likely the best Republican campaign of the cycle — turning what was expected to be a close race with Democratic former Gov. Ted Strickland into a likely blowout.

Indiana, though, delivered a blow to Democrats’ hopes. Former governor and senator Evan Bayh’s late entry was expected to turn the state into a guaranteed pickup — but scrutiny over his residency and lobbying work catapulted Republican Rep. Todd Young to victory, CNN projects.

Polls have closed in New Hampshire, Pennsylvania, Missouri and North Carolina, with those contests too close to call.

The House is also up for grabs Tuesday night, but Republicans are widely expected to keep their majority — even if their current 246-186 seat advantage is narrowed.

The races showed how Republicans fought to keep their party’s nominee, Donald Trump, at arm’s length.

In Pennsylvania, GOP Sen. Pat Toomey waited until nearly 7 p.m. ET — an hour before polls closed — to cast his own ballot.

Only then did he reveal for the first time that he voted for Trump. In Illinois, Sen. Mark Kirk, widely considered the Republican most likely to lose his seat Tuesday night, said he voted not for Trump but for David Petraeus.

Meanwhile, Clinton’s recent troubles, including the late focus on her email server, may have hurt Democrats down ballot.

House Democrats remain frustrated with FBI Director James Comey, who made a revelation 11 days before the election that the bureau was once again looking into emails potentially tied to its investigation of Clinton. Democrats believe his announcement provided down-ballot Republicans an opportunity to shift topics and attack Clinton, rather than defend Trump.

Minority Leader Nancy Pelosi said Tuesday that Comey “became the leading political operative in the country — wittingly or unwittingly.”

Pelosi blamed Comey for costing Democrats a shot at winning Republican seats across the country, saying he created “more of an obstacle that we hope to overcome,” but adding, “it’s difficult.”

Control of the Senate will be key to the early successes of a Trump or Clinton administration.

For Clinton, a Democratic Senate would represent a counterbalance to a House Republican conference that has fiercely attacked her on the campaign trail — even as some of its members have sought distance from Trump.

There’s also the more immediate impact: If Clinton wins and Democrats seize control of the Senate, Republicans could reverse months of resistance to confirming President Barack Obama’s Supreme Court nominee, Merrick Garland, and move quickly to approve him before Obama leaves office.

For Trump, a GOP Senate and House would, in theory, let Republicans swiftly pass legislation that reverses many of Obama’s policies.

In the Senate, Democrats need five Republican seats to get to the 51 needed for a majority. If the night ends in a 50-50 split, the new vice president would give his party the majority.

Republicans faced a daunting task at the outset of the 2016 election cycle: They held 24 of the 34 seats on the ballot — meaning many more to defend.

But strong GOP candidates have helped the party pull some contests in presidential swing states off the board.

In Florida, Sen. Marco Rubio’s loss in the presidential primary turned out to be the GOP’s gain, as he held onto that seat.

Still, the map features several toss-ups.

Wisconsin Republican Sen. Ron Johnson was expected to be an easy target for Democrats, but his race with Russ Feingold has tightened in recent days.

Republican senators in New Hampshire — where Ayotte faces Democratic Gov. Maggie Hassan — and Pennsylvania, where Toomey is opposed by McGinty — could see their fates tied to Trump’s performance Tuesday.

Two more Republicans who were expected to be safe — Missouri Sen. Roy Blunt and North Carolina Sen. Richard Burr — have fumbled, finding themselves in much more competitive races than expected.

In North Carolina, Burr’s late fundraising start — combined with the Clinton campaign’s focus there — allowed Democrat Deborah Ross to turn the state into a toss-up.

And in Missouri, Secretary of State Jason Kander has run one of the strongest Democratic campaigns — punctuated by a TV spot that featured the military veteran rebutting attack ads from the National Rifle Association by assembling an AR-15 rifle while blindfolded. If Blunt survives, he could be the rare Senate Republican who was helped by Trump, who is expected to carry Missouri easily.

Republicans, meanwhile, are only seriously challenging for one Democratic-held seat — Nevada, where Democrat Catherine Cortez Masto is squaring off with Republican Rep. Joe Heck for the seat being vacated by retiring Senate Minority Leader Harry Reid.

Though Tuesday night’s contests are likely to make clear which party will control the Senate, there could be some loose ends to clean up.

Georgia requires its victor to reach 50%. If neither Republican Sen. Johnny Isakson nor Democratic challenger Jim Barksdale reach that mark — which is possible, with a Libertarian in the race — the two would advance to a January runoff, though Isakson would be favored in the GOP-leaning state.

In Louisiana, a “jungle primary” system means there will be a runoff in December for the top two finishers. However, whichever Republican is the highest-performing finisher Tuesday is all but certain to win that runoff.

If Clinton and running mate Tim Kaine win, his Virginia Senate seat would open up. Democratic Gov. Terry McAuliffe would appoint a temporary replacement — and a permanent replacement would be selected in a 2017 special election.