The first results will start rolling in at 6 p.m., when some polls close in Indiana and Kentucky, but the telling moment Tuesday night will come two hours later, when voting ends in the Florida panhandle.
GOP presidential nominee Donald Trump’s path to the White House is virtually impossible without winning Florida, and if Democratic candidate Hillary Clinton can hold the state, she’s all but guaranteed a return trip to 1600 Pennsylvania Ave.
“If Hillary wins Florida, then most likely Hillary is going to be the next president of the United States, and it is going to be a short night,” said Aubrey Jewett, a political science professor at the University of Central Florida in Orlando. “If Trump wins Florida or is ahead in Florida, then the race can go well into the evening, and he certainly has a shot of winning overall.”
Florida also will provide early signs about whether Republicans are on course to retain control of U.S. Senate and if the party’s large majority in the U.S. House of Representatives is in jeopardy.
With a win by Mr. Trump in Florida, which he considers a second home because of his landmark Mar-a-Largo Club in Palm Beach, the outcome of the presidential race likely will not be decided until at least after the polls close at 9 p.m. across the Rust Belt states of Michigan and Wisconsin.
Flipping one of those typically blue states to red could be necessary to get Mr. Trump, the Republican nominee, the 270 electoral votes required to win.
The polls also close at 9 p.m. in swing state Colorado, where Mrs. Clinton has consistently led and where a win by Mr. Trump could signal a GOP landslide.
Along with Florida, the polls close at 8 p.m. in battlegrounds Pennsylvania and New Hampshire. Victories for Mr. Trump in these states — especially Pennsylvania — would confirm that the race is tilting toward the billionaire businessman.
Otherwise, Mrs. Clinton’s firewalls are holding. Mrs. Clinton has held a wide lead throughout the campaign in Pennsylvania, which hasn’t gone Republican since 1988 but was targeted by Mr. Trump as the linchpin of his plan to put Rust Belt states in the GOP column.
In the final days of the campaign, the Pennsylvania race tightened to a virtual tie in polls.
Back in the Sunshine State, Americans also will get an early clue about whether the Democratic Party is poised to seize control of the U.S. Senate.
Republican incumbent Sen. Marco Rubio is in a strong position in the state, and an upset win by Democratic challenger Rep. Patrick Murphy could spell doom for the GOP majority.
“If Patrick Murphy can pull the upset on Marco Rubio, it almost certainly means there is a Democratic wave election coming,” Mr. Jewett said.
Senate Republicans now hold a 54-46 majority in the 100-member chamber. Democrats need a net gain of five seats to take over the chamber, or four seats if Mrs. Clinton wins the Oval Office and Democratic vice presidential nominee Tim Kaine becomes the tie-breaking vote.
Florida is among eight states with close races for the U.S. Senate. But the closest races where Republican seats are in jeopardy are Indiana, Missouri, North Carolina, New Hampshire and Pennsylvania.
The results in Florida also will give an early indication of whether the GOP majority in the House is at risk.
Rep. John L. Mica, who has represented Florida’s 7th Congressional District since 1993, is in one of the toughest re-election battles of his career. If he loses the Orlando-area seat to Democrat Stephanie Murphy, a former Defense Department analyst, it should set off alarm bells for House Speaker Paul D. Ryan.
In the 435-member House, where a party needs 218 seats to claim the majority, Democrats enter the election with 186 seats, meaning they need a net pickup of 32 seats — a rout by any measure, but not outside the realm of possibility — to seize the speaker’s gavel.
Another House race to watch in Florida is the rematch of Republican Carlos Curbelo and Democrat Joe Garcia in the 26th Congressional District. Mr. Curbelo unseated Mr. Garcia in the South Florida district in 2014, and Democrats have poured more than $2 million into taking it back.
Polls show the race is extremely close. A win by Mr. Garcia could indicate that Mr. Trump is a drag on down-ballot Republicans and a rout is possible.
Before Florida, results will begin to be tallied in several key states in the presidential race.
The polls close at 7 p.m. in Georgia, Indiana, Kentucky, South Carolina, Vermont and Virginia.
Mrs. Clinton has been running in the lead and looking for a decisive victory in Virginia, a once reliably red state that appears to have fully realigned since turning blue for President Obama in 2008 and 2012.
The race in Georgia has been unusually close, but a win there for Mr. Trump would also signal that he is winning where Republicans need to run strong.
Another two crucial battleground states close the polls at 7:30 — North Carolina and Ohio.
Mr. Obama narrowly won North Carolina in 2008 but lost there to Republican Mitt Romney in 2012. The Clinton campaign has poured resources into the state, including repeat visits by Mr. Obama and first lady Michelle Obama, in an effort to deny Mr. Trump the state’s 15 electoral votes.
A win for Mr. Trump in North Carolina would confirm that Mrs. Clinton’s firewall had broken. If Mr. Trump couples that with a victory in must-win Ohio, he will have opened up a clear path to the White House.
North Carolina and Ohio also weigh heavily on the battle for control of the Senate.
North Carolina’s contest between Republican incumbent Sen. Richard Burr and Democratic challenger Deborah Ross has been one of the closest races in the country.
In Ohio Republican incumbent Sen. Rob Portman holds a double-digit lead in state polling over former Gov. Ted Strickland, a Democrat. If Mr. Portman does not cruise to an easy victory, as expected, that could be a bad sign for Mr. Trump and Senate Republicans across the country.