As early election results trickled in Tuesday night, Republican U.S. Rep. Robert Dold had a healthy lead over Democratic challenger Brad Schneider in the suburban 10th District.
Within an hour of the polls closing, thousands of votes separated the two candidates, and it looked like Dold was headed to a second term.
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But Dold, of Kenilworth, couldn't keep the pace. Precinct by precinct, Schneider -- a first-time candidate from Deerfield -- closed the gap and eventually pulled ahead, first in Cook County and then in Lake.
Early and absentee voting in Lake County put Schneider over the top.
When it was over, Schneider had eked out a victory with a 2,547-vote margin, unofficial results showed. That's less than 1 percent of the total votes cast in the race.
It's the first time a Democrat has won the 10th District seat since the 1970s.
A big early-voting push was part of the Democratic playbook, said state Sen. Terry Link, the party's chairman in Lake County.
Of the more than 100,000 people who voted early in the district, Link and other Democratic campaign organizers estimated 60 percent likely supported Schneider, based on geography and other factors.
"We knew where they were," Link said. "This is what we were banking on."
If Democratic voters could keep it close on Election Day, Link knew the early votes would give Schneider the win.
"This is why we keep preaching on early voting," Link said. "We tell candidates, you've got to work. ... It's not just one day anymore."
He also touted endorsements by the pro-environment Sierra Club and other activist organizations as helping to tip the scales for Schneider.
In previous 10th District elections, the Sierra Club backed Republican Mark Kirk. This time, they went for Schneider, even though Dold had proclaimed himself in favor of environmental protection, particularly regarding the Great Lakes.
"I think those were huge factors in this election," Link said.
His Republican counterpart, Bob Cook, primarily attributed Schneider's upset win to the negative campaign the Democrats ran against Dold.
In TV ads, emails, fliers and public appearances, they painted Dold as an ally of the conservative Tea Party, even though Dold long has campaigned as a social moderate.
They criticized his votes on the budget, the environment, abortion, gay rights and other divisive issues.
And despite being dramatically outraised and outspent by the Dold campaign, it worked.
"In the end, they convinced just enough people," Cook said.
The Republicans weren't afraid to get into the mud, either.
In debates and interviews, Dold went after Schneider for not releasing his family's income tax returns, and he questioned Schneider's business background following the release of disclosure reports that showed the Democrat's consulting firm had garnered little income in recent years.
Those attacks were repeated in TV commercials and web videos from Dold and the Republican Party.
But the effort wasn't as effective.
"I think they had a better attack campaign," Cook said of the Democrats.
The new congressional map likely was a factor, too, Cook said. Ahead of the election, the 10th District -- already notoriously independent -- was reshaped to include more traditionally Democratic areas than it previously contained.
The updated boundaries made the 10th District the most left-leaning district represented by a Republican in the nation, some political analysts said.
And were it not for the early voter turnout, Dold might have held onto the seat.
"The fact that (Dold) was leading most of the night showed what a great campaign he ran," Cook said.