President Joe Biden and ex-President Donald Trump held duelling rallies as the election to reshape Congress entered its final campaign day.
Mr Biden and Mr Trump made their last-minute pleas to voters in New York and Florida respectively.
Momentum has shifted recently towards Republicans, who are trying to wrestle both chambers from Democratic control.
They are favourites to win control of the House of Representatives but the Senate is a toss-up, polling suggests.
Winning one chamber would severely hinder President Biden’s legislative agenda.
With the campaign in its final day on Monday, his party is braced for losses even in parts of the country where Democrats usually do well.
He spoke at a rally in New York on Sunday to support Governor Kathy Hochul, who is fending off an unexpectedly stiff challenge from Trump-backed Republican candidate Lee Zeldin.
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She has received last-minute help from some Democratic star names – former President Bill Clinton, Vice-President Kamala Harris and former Secretary of State Hillary Clinton.
In his speech, Mr Biden called the election “an inflection point” that will determine the next 20 years.
He told voters at Sarah Lawrence College that they are choosing between two “fundamentally different visions of America”.
Meanwhile his predecessor in the White House, Mr Trump, was in Miami where he spoke for over an hour, hammering Democrats for leading the country towards “communism”.
“Democrats want to turn America into communist Cuba or socialist Venezuela,” Mr Trump told the audience members.
“To every Hispanic American in Florida and across the land, we welcome you with open open arms to our [Republican] party,” he continued.
Mr Trump also continued to hint that he may run for president again in 2024, telling voters to “stay tuned” for his rally on Monday in Ohio.
Polls suggest that Democrats are likely to lose their majority in the House of Representatives.
Control of the Senate will probably rely on the results of extremely tight races in Pennsylvania, Georgia and Nevada.
Over 40 million votes have already been cast during the early-voting period so far, experts say, overtaking the total number of early votes in 2018.
Mail-in ballots normally take longer to tally than votes taken in person, leading to a high likelihood that several races will be too close to call on Tuesday night.
Several key battleground states, like Wisconsin and Pennsylvania, ban election officials from beginning the count until election day.