MANASSAS, Va. — It ended here.

Before a crowd of 100,000 in the fields of northern Virginia on Monday night, Democratic presidential nominee Barack Obama closed among the most epic presidential campaigns in American history.

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“Virginia, I just have one word for you,” he told the assembled masses here. “Tomorrow.”

Indeed, only Election Day remains. Yet it dawns with uncertainty: inclement weather, polling irregularities and precinct stations short on ballots threaten the campaign’s vaunted get-out-the-vote efforts. Despite all that, Obama asked his voters to cast their votes.

“You can’t let that stop you, you got to wait that line, you got to vote, you got to take your friends, take your neighbors,” he said. “We can’t stop. Not now, not when there’s so much at stake. We’re going to change America starting tomorrow.”

It was no surprise that Obama chose this Washington, D.C., exurb in as his final campaign stop. Virginia is a battleground state — one of a few whose voters will effectively choose America’s 44th president tonight.

More than 2,000 miles away, Republican presidential hopeful John McCain made his closing arguments before a Prescott, Ariz., crowd. His Monday rally capped a whirlwind day of campaigning that saw McCain touch down in battleground states Indiana, Florida, Pennsylvania, Nevada and New Mexico before returning to his home state of Arizona.

“It’s been a long, long journey and we’ve got one more day,” McCain told supporters at his last event. “I need you to do what’s necessary so that we not only take Arizona in a big way but also get this country back on track again.”

More than a few Yale students disagree with McCain. The most recent News poll of the undergraduate student body showed McCain trailing Obama by 69 percentage points. That support has translated into a deep wave of on-campus support for the candidate. Hundreds of students traveled to Pennsylvania and New Hampshire with Obama-backing organization “Yale for Change” this weekend to canvass neighborhoods of voters whose decision today will have an outsized influence on the final results.

The Yale College Republicans, meanwhile, have turned their attention to keeping Republican Rep. Chris Shays in his seat in the 4th Congressional District — even after Shays distanced himself from the Republican candidate last week.

On Election Day, a few of the hardiest Eli volunteers will skip class to drive to battleground states for one final push. Meanwhile, their colleagues in New Haven will phone bank, calling likely voters in the battleground states of Florida, Ohio and Virginia.

The last on that list is crucial. If the state tips toward the Democrat, the election could be called early Tuesday night. The Elis know this, and so does Obama.

* * *

Once, Manassas witnessed one of the first battles of the Civil War. It finds itself, 147 years on, in the no-man’s land of another — one not quite as long but almost as bitter. An influx of new residents has turned the traditionally red countryside surrounding Manassas, a town of 35,000, increasingly purple over the last few years. A Democratic governor sits in the governor’s mansion in Richmond. Virginians are virtually certain to send their second Democratic senator — former Gov. Mark Warner — to Washington on the strength of today’s vote.

And at the Obama rally here Monday, the excitement was palpable.

Outside Prince William County Fairgrounds, a throng of thousands trailed along as vendors sold makeshift buttons and T-shirts.

Áine McDonough, 24, and Fiona O’Byrne, 25, were among the throng. McDonough, wearing an Obama T-shirt, is a graduate student at the University of London, O’Byrne an actuary at Trident Benefit Consulting. Both from Ireland, the two are on a vacation in Washington, D.C. — timed to coincide with the election — and came to witness Obama’s final rally in Virginia.

“It’s the whole sense of history kind of thing,” McDonough said.

Added O’Byrne: “Yeah, the impact this has on the world is massive.”

Inside, the fairgrounds were a sea of blue signs for change. A mob of frenzied supporters. A junior senator from Illinois who promised his supporters change and hope.

“Yes we can! Yes we can! Yes we can!” bellowed the crowd. “Obama! Obama! Obama!”

Kim Love arrived at the rally around 2:00 p.m. Monday. She is a veteran of the Gulf War; her daughter served recently in the Iraq War. By the time Obama took the stage 90 minutes late, she had waited eight and a half hours to see the Democratic nominee. And Love, a longtime Virginia resident, has already voted ­— early — for the senator.

“Virginia has been red as long as I’ve been here. For Virginia to possibly be blue,” she said, “is so exciting.”

* * *

Virginia could be something of a firewall for Obama tonight.

“There are a lot of important decisions to be made and no state is going to be more important in this election than this state right here,” Obama was quoted by CBS News as saying before a rally at James Madison University last week. “The great state of Virginia.”

If McCain’s inroads into Pennsylvania pay off, and if Ohio is again a disappointment for the Democrats, the hopes of hundreds of Obama supporters nationwide and here on campus could depend on whether a purple state swings blue or red.

For the last 44 years, Virginia has bequeathed its 13 electoral votes to the Republican presidential nominee every election. Those votes helped Yale alumni George H. W. Bush ’48 and George W. Bush ’68 to victory in 1988 and 2000. But they also threatened the candidacies of Bill Clinton LAW ’73 and ultimately helped to end that of John Kerry ’66.

On Saturday, McCain made a final, emotional pitch to voters in this state, urging them to fight on to the final minutes of the campaign.

“We never give up. We never quit. We never hide from history. We make history,” he boomed before a Springfield, Va., crowd that day. “So let us get out and win this election and get our country moving again.”

The Arizona senator has since moved away from this state, trusting his field organizers and a track record of conservative voting to keep the state in his column.

But Monday night, Obama was here, behind a lectern just 33 miles from 1600 Pennsylvania Avenue. There was a moment Monday when his appearance at in Virginia seemed an open question: His grandmother died of cancer Monday in Honolulu.

Still, his speech at Manassas Park was focused on tomorrow.

“I just want to say whatever happens I’ve been deeply humbled by this journey,” he said Monday night. “You’ve filled me with new hope for our future, and you’ve reminded me about what makes America so special.”

After his appearance here Monday, after 20 months of advertising and fundraising, speechmaking and glad-handing, the man who captured the hearts of many Elis and many more voters across the country has no more campaign stops to make. Now, he waits.

Paul Needham contributed reporting from Prescott, Ariz.