Update: 8:38 p.m.
The first big result of the night (for Senate races) is in: Evan Bayh has lost his bid in Indiana to return to the Senate.
It's a stunning loss for Democrats, who thought they were heavy favorites to win the race the moment the former senator in July, in a shock move, decided to re-enter public life run for his old seat.
But the campaign went bad from almost the moment he started his campaign, when Republicans began raising questions about everything from his residency in the Hoosier State to the more than $200,000 he spent in taxpayer money on flights during his former tenure in the Senate.
Update: 8:20 p.m.Marco Rubio's re-election victory was expected by both parties — Democratic groups like Senate Majority PAC, after all, decided not to spend hardly anything in the race in its final months.
But the incumbent's win, especially coupled with Bayh's early struggles in Indiana, suggests that no matter what happens the rest of the night, Senate Democrats can win no more than 52 seats.
The party, which has already picked up a seat in Illinois, expected it could win in only five other states held by a GOP senator: Wisconsin, Pennsylvania, New Hampshire, North Carolina, and Missouri (along with holding the retiring Harry Reid's seat in Nevada).
Its the first Senate pickup for the Democratic Party on a night in which they need to claim a net of four seats to control the legislative body next year.
Meanwhile, Republicans say they feel good about the Indiana Senate race, where Democratic nominee Evan Bayh is struggling to over-perform Hillary Clinton by a large enough margin to win in a red state. Indiana isn't an essential place to win for Democrats, but party operatives still considered it a winnable race.
Update: 7:30 p.m.
In an expected but nonetheless huge victory for Republicans, Sen. Rob Portman of Ohio won re-election Tuesday, according to The Associated Press.
He easily defeated Democratic nominee Ted Strickland, the former Ohio governor who Democrats once billed as a top recruit.
Strickland's campaign collapsed because of a lack of money and unrelenting attacks from the GOP, which argued he had lost touch with his roots in the state since losing his gubernatorial re-election in 2010.
Portman is widely credited with running the best Senate Republican campaign of the election.
Democrats have a path back to controlling the Senate without Ohio, but Strickland's loss still leaves on the table a race the party once believed it could win.
Update: 7:00 p.m.
One of the great mysteries of the 2016 election has been answered: Republican Sen. Pat Toomey of Pennsylvania supports Donald Trump's bid for the presidency.
The vulnerable first-term incumbent announced his vote late this evening, according to Laura Olson of The Morning Call.
Toomey had kept everybody in suspense for months, repeatedly saying that he had not yet decided if he would back the GOP presidential nominee. (He had long since ruled out voting for Hillary Clinton).
He was the only Republican senator this year to decline to say who he supported for president. It was part of a strategy from the incumbent to appeal to voters in vote-rich southeast Pennsylvania while not alienating Trump's core supporters.
Democratic nominee Katie McGinty repeatedly criticized Toomey for declining to say how he'd vote for president.
The two candidates are locked in one of the tightest Senate races in the country.
Senate Democrats had once hoped that a blue-state heavy map and Donald Trump’s toxic presence on the Republican ticket would offer them a glide path back to the majority.
Things haven’t worked out that way — but the party might be on the verge of taking control of the legislative body anyway.
Democrats must win a net of four seats Tuesday to reach their goal, no sure bet even if Hillary Clinton is likely to win the presidency (as down-ballot operatives in both parties believe). Republicans and Democrats are fighting over eight battleground states — Wisconsin, New Hampshire, Pennsylvania, Missouri, Indiana, North Carolina, Florida, and Nevada — each side has targeted with a late spending surge.
But a Republican represents all but one of those states, Nevada. And public polls suggest that Democrats have inched ahead in enough of them that they could cobble together at least 50 seats next year.
In Pennsylvania, for instance, GOP Sen. Patrick J. Toomey trails Democratic nominee Katie McGinty in 10 of the last 12 public polls. Some Republicans also remain skeptical that Wisconsin Sen. Ron Johnson, despite a late and unexpected comeback, will ultimately defeat Democratic former Sen. Russ Feingold.
And in the early vote in Nevada, Democrats have seized an advantage comparable to the one President Obama held in his 2012 re-election race, when he won the state comfortably.
The landscape was never going to be easy for Republicans in 2016. The party had to defend seats in eight states that President Obama won twice. (Another, North Carolina, Obama won in 2008 and lost narrowly four years later.) Turnout patterns in presidential years, which usually show a spike in participation by younger voters and racial minorities, also favor Democrats.
Then the GOP had to grapple with Trump, whose unexpected rise saddled the party’s Senate candidates with the least popular presidential nominee ever, whose penchant for incendiary rhetoric continually put them on the defensive.
Under those adverse conditions, Senate GOP strategists express surprise they even have a chance to hold the Senate. And even if Republicans do lose the majority, the party believes it has minimized its potential losses.
Races in Arizona and Iowa (represented by Sens. John McCain and Charles E. Grassley, respectively), for instance, never became competitive despite early optimism from Democrats that they could make both seats competitive.
And one former battleground — Ohio — unraveled for Democrats when their nominee, the former Gov. Ted Strickland, wilted under an onslaught of spending from GOP Sen. Rob Portman and allies. A survey released Sunday from The Columbus Dispatch found Portman winning by 21 points.
The battle for the Senate won’t technically end Tuesday. The top two vote-getters in the Louisiana Senate race — which polls indicate will be GOP state Treasurer John Kennedy and either Republican Rep. Charles Boustany Jr. or Democratic Public Service Commissioner Foster Campbell — will enter a Dec. 10 runoff.
Sen. Johnny Isakson of Georgia might also enter a runoff if he fails to reach 50 percent of the vote on Tuesday.
The GOP is expected to win runoffs in both states with ease.