For all the expectations about a late and possibly nail-biting night Tuesday, one key question about the presidential election should be clear early in the evening: just how well President Obama performed among early voters in the swing state of Ohio.

Polls in the Buckeye State will close at 7:30 p.m. Eastern time on Tuesday, and the Ohio secretary of state’s office said counties are prepared to quickly post tallies from absentee and in-person early voting. A spokesman said those numbers should be available as early as 7:45 p.m. to 8 p.m.

What does that mean for those following at home?

Public polling leading up to Election Day has indicated that early voters in Ohio broke for Obama by large margins. That means we should expect the president to take an early and potentially sizable lead in the all-important battleground when the early-vote tally is posted.

The question will be just how far ahead Obama is as county election boards begin to post results from Election Day. Those returns are expected to favor Republican challenger Mitt Romney.

(If Romney is ahead after early votes are posted — or if the two are very close — there’s a good chance that Romney will win the state and its 18 electoral votes.)

Another key number to watch in Ohio is how many provisional ballots have been cast in a state that has experienced major legal wrangling over voting rules.

There were 150,000 such provisional ballots cast in 2008. There could be at least that many this year — including those cast by voters who requested an absentee ballot but didn’t return it. State rules say such people cannot use standard ballots on Election Day and must vote provisionally.

Matt McClellan, a spokesman for Secretary of State Jon Husted (R), said election boards have been instructed to report the number of provisional ballots cast in their counties before concluding their work Tuesday or early Wednesday.

That means that by the end of the Election Day counting, it will be known which candidate is ahead and how many provisional ballots remain to be counted.

Provisional ballots tend to be used more often by low-income and transient voters, and both sides assume they will break strongly for Obama.

That means Romney could hold a small lead in Ohio at the end of the counting on Tuesday and still lose the race there — possibly even decisively — if several hundred thousand provisional ballots remain to be counted.

Under that scenario, the public should prepare for a long wait. Ohio rules say provisional ballots cannot be counted until 10 days after an election.

Election watchers will be looking at the places Obama won in 2008. He captured Ohio by racking up a huge margin of victory in the Democratic strongholds of Cleveland (Cuyahoga County), Toledo (Lucas County) and Columbus (Franklin County).

In 2008, Obama won nearly 50,000 more votes in Franklin County and almost 10,000 more votes in Cuyahoga than Sen. John F. Kerry (D-Mass.) did four years earlier. He doesn’t need to do that well again to win the state, but he does need to swamp Romney in those urban areas.

In Cuyahoga, for instance, if results show that Obama is winning by a similar margin to 2008 (when he took nearly 70 percent of the vote) and if a similar number of people have voted (665,117), things probably will look good for him this year, too.

Obama became the first Democrat since 1964 to carry Hamilton County, home to Cincinnati. Republicans feel confident that they can take back the traditional GOP stronghold. If Romney is winning Hamilton as the night wears on, that’s probably a good sign for the Republican.

Election watchers also will be keeping an eye on the places Obama lost in 2008.

Part of his formula then involved winning more votes than Kerry did in 2004, even in counties that strongly supported Sen. John McCain (Ariz.), the Republican nominee.

Take Delaware County, a reliably Republican suburb of Columbus. McCain won 59 percent of the vote there, but Obama managed to identify and turn out his own supporters in the traditionally red area. Even though he lost the county, Obama racked up 9,600 more votes in Delaware County than Kerry did four years earlier.

If Obama’s count in Delaware County looks more like Kerry’s tally of 27,000 — and less like the 36,652 votes he won in 2008 — that will be a bad sign for Obama and good news for Romney.

Likewise, in the Cincinnati suburbs, Obama will almost certainly lose in Warren and Butler counties. But election watchers will be looking to see whether his vote tallies look more like his performance in 2008 (Warren: 33,398; Butler: 66,030) or Kerry’s count in 2004 (Warren: 26,044; Butler: 56,243).