It’s finally here.

After hundreds of campaign rallies, billions of dollars in donations, a nonstop blitz of TV advertising and polling that’s been in full swing since summer, Election Day has arrived – and with it, voters’ midterm verdict on whether Republicans should hold or lose their grip on power in Washington.

Democrats have been working toward this day since the moment President Trump was sworn in. They’ve waited two long years for the shot to win enough seats in Congress to cripple the 45th president’s agenda.

In the end, the elections will go one of three ways: Republicans hold both chambers, Democrats take the majority in both, or the parties split the House and Senate. Either of the latter two scenarios would be problematic for Trump, immediately putting a damper on his agenda.

By the numbers, 35 Senate seats, 36 gubernatorial seats and 435 House seats are up for grabs on Tuesday. And the biggest names in both parties say the stakes couldn’t be higher.

“America is at a crossroads right now,” former President Barack Obama told supporters in Illinois over the weekend, saying it “might be the most important election of my lifetime, maybe more important than 2008.”

Trump himself has been campaigning non-stop in recent days, telling voters they must keep Democrats from taking back control of the House and Senate, or his agenda could be in jeopardy. The party of the president has historically lost seats in their first midterm elections.

“We have to win,” Trump told supporters Monday during a telephone town hall, ticking off accomplishments during his first years in office and warning everything he’s accomplished as president can be “undone and changed by the Democrats.”

Here’s the lay of the land on Election Day:

For Democrats, their best shot at winning back control of a body of Congress is in the House, where they need to net 23 seats for a majority. Whether they flip the House or not, Democrats are still expected to gain seats in the chamber, given the favorable political environment for Democrats.

Republicans currently hold a slim 51-49 majority in the Senate, and Democrats have long aimed to win back control. But it will be more difficult for Democrats to be victorious in the Senate by netting two seats in part because of the large number of incumbent Democrats running for re-election in states won by Trump in 2016.

Republicans currently control 33 governorships, with Democrats holding 16. But with 36 seats up this year, Democrats are likely to pick up at least some seats. According to the Fox News Gubernatorial Power Rankings, Republicans are favored to control at least 22 seats by the end of the night and Democrats are favored have at least 20 seats, with 8 additional toss-up races.

Election watchers could be in for a long time. The first polls, on the East Coast, close at 7 p.m. ET. After polls on the West Coast close at 11 p.m. ET, the last will close in Alaska at 1 a.m. ET.

Ahead of the vote, House Democratic leader Nancy Pelosi confidently predicted last week that Democrats would indeed win back the House.

“Let me say this. Up until today, I would’ve said, ‘If the election were held today, we would win,’” Pelosi said on “The Late Show with Stephen Colbert” Tuesday. “What now I’m saying is, ‘We will win.’”

But there are no guarantees about Election Day, even as generic congressional ballots show Democrats with the advantage over Republicans. While Democrats need to net 23 seats for a majority, the Fox News House Power Rankings indicate there are 29 “toss-up” races.

If Democrats are victorious, the Trump administration likely will be in for a challenging 2019. Should Democrats re-take the House, there are signs some in the conference may launch a push to impeach the president – though the leadership has not yet backed the effort. Pelosi has said impeachment is "not a priority," at least not until Special Counsel Robert Mueller announces the conclusions of his probe into the Trump administration's alleged dealings with Russia.

Trump, though, almost assuredly will face an avalanche of investigations into his tax returns, his payments to adult-film star Stormy Daniels and his relationship with Russia if Democrats win back the House – which comes with the much-coveted power to subpoena and launch congressional investigations.

PRESIDENT TRUMP'S CLOSING ARGUMENT: VOTE REPUBLICAN AND CONTINUE THE JOBS BOOM

A number of prominent anti-Trump Democrats also would be poised to assume control of key committees if Republicans lose control of the House.

Rep. Maxine Waters, D-Calif., likely would be elevated to chairwoman of the House Financial Services Committee, where she currently serves as ranking member. Another top California Democratic representative, Adam Schiff, would likely rise to chair the House Intelligence Committee, where he is now the ranking member. And Pelosi would look to regain the speakership.

Still, if Democrats win the House – but Republicans hold the Senate – their legislative priorities likely would stall.

In the Senate, the Fox News Senate Power Rankings lists Florida, Indiana, Missouri, Arizona and Nevada as the five most competitive races. Of those races, Democrats in three states won by Trump are defending their seats.

Republicans feel more confident about keeping control of the Senate – or possibly adding to their majority. But the threat to the Trump agenda is real if they lose control. Democrats have a narrow path to the majority if they run the table in the toss-ups and pick off one GOP-favored seat, like Texas or Tennessee. If Democrats win control of both houses of Congress, they could send legislation to Trump’s desk. And, in the Senate, they would have the votes to block any of Trump's nominees – coming after Justice Brett Kavanaugh’s brutal confirmation fight last month.

On the day before America votes, Trump hopscotched to rallies in three battleground states -- in Ohio, Indiana and Missouri.

In Ohio, Trump campaigned for a slate of Republicans, including Jim Renacci in his bid to unseat Democrat Sen. Sherrod Brown. Trump won the state with 52 percent of the vote in 2016 but the Fox News Senate Power Rankings lists that contest as “likely Democrat.”

Later Monday, the president flew to Fort Wayne, Indiana, where he is hoping to give Republican Mike Braun an assist to defeat incumbent Democratic Sen. Joe Donnelly. He then traveled to Missouri to lend a hand to Josh Hawley in his campaign to beat incumbent Democratic Sen. Claire McCaskill.

Meanwhile, in state capitals across the country, Republicans have control of a majority of governorships, holding 33 to the Democrats’ 16 seats.

Democrats are likely to pick up at least some seats, with 36 gubernatorial seats up for grabs this year. According to the Fox News Gubernatorial Power Rankings, 15 of the contested seats lean Republican, 13 lean Democrat and eight are tossups.

For Republicans, holding on to control of state houses is important, considering that’s where real policy changes may take place if Democrats win control of Congress and Washington effectively deadlocks.

Among the most-watched races: Wisconsin GOP Gov. Scott Walker is locked in a tight re-election battle with Democratic challenger Tony Evers and Tallahassee Democratic Mayor Andrew Gillum is facing off against Trump-backed GOP Rep. Ron DeSantis in Florida.

Though the political environment seems to favor Democrats, Trump argued Monday that Republican voters are energized heading into Tuesday’s vote, saying he hasn’t felt such “electricity” since he was elected president in 2016.

"Tomorrow, with your votes, you can stop the radical resistance in its tracks," Trump told the crowd in Cleveland.

Fox News’ Gregg Re contributed to this report.