Democrats are well positioned to take back the House of Representatives for the first time since 1994, and also have a shot at capturing the Senate.
But with razor-thin margins separating candidates in several key contests, and no laws requiring citizens to vote, both major parties were relying on well-honed mobilization efforts to rally their base voters to the polls.
Democrats need a net gain of 15 seats out of the total 435 in play to control the House. They hope for a gain of six Senate seats out of the 33 at stake to give them the edge in the 100-member upper chamber.
At stake is control of the US legislative agenda for the final two years of US President George W. Bush's presidency.
Top Republicans who rallied party members were buoyed by recent polls that showed they had narrowed the gap in several national voter surveys.
"New polls say our party is heading into Election Day with strong momentum," said Republican National Committee chairman Ken Mehlman in a memo to the party faithful, adding that Republicans could be heartened by "major gains" made in opinion polls over the past day or so.
Mehlman said the gains were likely even more substantial than polling suggests, since the vaunted Republican get-out-the-vote machine operates largely under the radar.
Key among those efforts is the now-famous "micro-targeting" strategy that identifies likely Republican supporters who live in liberal enclaves, and then works to get them to the polls.
Mehlman said loyal Republican volunteers over the weekend had succeeded in reaching millions of possible supporters.
"By and large, this effort will have its impact on Election Day and will not show up in most public opinion polls," he said.
Republicans are determined to confound Democratic predictions of sweeping election gains with their better-funded voter turnout machine, which led Bush to re-election in 2004.
Democratic Senator Chuck Schumer, who runs election efforts for his party, was hopeful, but not willing to predict Democrats' prospects for taking back control of the upper chamber.
"We're right on the edge of taking back the Senate," he said Sunday, but added, "I wouldn't open up the champagne or do the high-fives."
Democrats have incorporated some of the techniques that Republicans have used to good effect in the past, but also relied on tried-and-true recruitment and outreach methods used by labour unions for decades.
Union officials said the largest and most powerful US labour federation, the AFL-CIO, has rallied some 100,000 volunteers to make a planned five million phone calls in the last few days of the campaign.
"Working people have had enough of disappearing jobs, unaffordable health care, stagnant wages, retirement insecurity, corrupt politicians and a congressional majority that rubber-stamps President Bush's anti-worker policies," the AFL-CIO statement said.
"On November 7, we can take back America for working families --if we vote."