Democrat Joe Biden is leading in the race for the White House and his party is favoured to win the Senate as well, in an underlooked contest that could have far-reaching ramifications for years to come.
President Donald Trump had told Republican donors at a closed-door meeting that it would be "very tough" for his party to hold the Senate, where it now has a 53 to 47 majority, the Washington Post reported this week.
Even if Mr Trump is re-elected, a Democrat-controlled Senate and House could thwart his legislative agenda as well as impeach him again, although removing him from office requires a far higher threshold of 67 senators than they will likely be able to muster.
But should Democrats regain the White House, flip the Senate and keep the House of Representatives, they would have unified control of the federal government for the first time since 2011, empowering them to pass broad reforms in healthcare, criminal justice and environmental protection.
Many more options would also be open to the Democrats that could fundamentally shift the balance between the two parties, including expanding voting rights and turning the District of Columbia and possibly Puerto Rico into America's 51st and 52nd states.
This goal is well within reach for the Democrats if Mr Biden wins and they gain a minimum of three seats to control the Senate.
Thirty-five Senate seats are up for grabs and the Republican Party is defending 23 of them. But it is vulnerable in 12 races, including seven which the non-partisan Cook Political Report rates as toss-ups: two in Georgia, and one each in Iowa, Maine, Montana, North Carolina and South Carolina.
In contrast, the Democrats have 12 seats to defend and are vulnerable in only two: Alabama and Michigan. They are also widely seen by pollsters as likely to retain control of the House and even expand their current majority.
Vulnerable Republican senators have been weighed down by a dip in the President's national approval ratings, stemming largely from his handling of the pandemic.
And they are trying to distance themselves from Mr Trump in their comments and election advertisements, they added.
"Republican incumbents seem not to have been able to establish records of their own that would commend them to voters, not only to those who support Donald Trump but to some who don't," said conservative political analyst Michael Barone at an American Enterprise Institute panel on Oct 20.
But Senate Majority leader Mitch McConnell said the nomination of conservative judge Amy Coney Barrett to the Supreme Court had boosted the re-election chances of Republicans in tight races. "I think it's a 50-50 proposition," he said of keeping the Republican Senate majority, in an interview with The Washington Post on Wednesday.
Democrats' dreams
Progressives are already calling for broad reforms - some of which were Bills passed by the Democratic House but not advanced by the Republican Senate this term - should they win the Senate.
Among them are the electoral reforms detailed in the For the People Act, the first Bill passed by the current iteration of the House of Representatives in January 2019. They include diluting the power of big money in politics and curbing partisan gerrymandering.
The Bill's efforts to expand voting rights, including making election day a federal holiday and creating a national automatic voter registration programme, are aimed at boosting voter turnout.
The Democratic Party, which dominates the more populous coastal states, is also disadvantaged by the Senate's structure of giving every state two seats regardless of its population.
So, some Democrats favour granting statehood to the District of Columbia, which has three electoral college votes but no representation in the Senate or the House of Representatives, even though its population of 700,000 is larger than Wyoming's 580,000 and Vermont's 630,000.
The District leans heavily blue, which would in theory give Democrats two more votes in the Senate and likely one in the House. The Democratic House passed a Bill making DC the 51st state this summer, but the Republican Senate did not take it up.
The partisan fight over Justice Barrett's recent confirmation has also prompted calls for a Democratic Senate to raise the number of Supreme Court justices.
"Remember that Republicans have lost six of the last seven popular votes, but have appointed six of the last nine justices," progressive lawmaker Ilhan Omar tweeted. "By expanding the court we fix this broken system and have the court better represent the values of the American people."
Since 1869, the number of Supreme Court justices has been nine, although the number changed six times before that.
A YouGov poll earlier this month found 47 per cent of voters oppose packing the court to 34 per cent who favour it, and Mr Biden has been hesitant to state where he stands, saying he would instead set up a commission to study options to overhaul the court.
The sweeping reforms will also not have the support of everyone, with moderate Democrats, who fear it may hurt their re-election chances, less likely to be keen on them than progressives, said commentators.
