Republicans are expected to keep their majority in the Senate and forecast to gain one seat in the upper chamber.
In the Miami area, former Clinton administration Cabinet member Donna Shalala defeated television journalist Maria Elvira Salazar in a costly, roller-coaster contest.
Early returns showed Democrats poised to pick up the 23 House seats they needed to gain a foothold in Congress from which to counter Trump.
"We have long cautioned against assuming the House was a done deal for the Democrats, and we don't think readers should be stunned if things go haywire for Democrats tomorrow night", Crystal Ball editor-in-chief Larry Sabato and managing editor Kyle Kondik wrote.
During his final campaign rallies in Ohio, Indiana and Missouri, Mr Trump returned to his key campaign issues, insisting that Democrats would damage the economy and allow more illegal immigration.
And in West Virginia - where Trump is wildly popular and campaigned heavily for Republicans - the reelection of Democratic Sen. He predicted an "invasion" from the migrant caravan making its way toward the USA and decried the "radical" agenda of speaker-in-waiting Pelosi.
Political gridlock between the White House and Congress could hinder Trump's pro-business agenda and raise concerns about United States political instability, but investors may have already priced this in.
According to Michael McDonald of the US Elections Project, 38.4 million Americans cast their ballots early ahead of this election, compared with 27.4 million in the 2014 midterm.
Almost 40% of voters cast their ballots to express opposition to the president, according to a national survey of the electorate, while one-in-four said they voted to express support for Mr Trump. It was a deliberate strategy to expand the playing field to about 80 districts, stretching beyond college-educated voters in the suburbs into regions where the party has seen its fortunes fade.
‘Unsafe’ close pass by Russian fighter jet ‘jostles’ US Navy aircraft
At the time, the US State Department accused the Russians of "flagrantly violating existing agreements and worldwide law". The Navy released a short video clip Monday of the Russian jet's afterburner flaring alongside the USA aircraft.
South Florida voters cast their ballots at a polling center in Miami, Florida on November 6, 2018. Hundreds of millions were spent by the parties, supplemented by more money from outside groups, to frame the debate. The Republicans have a majority in the House with 235 congresspeople to 193 Democrats. He's running for Senate against the Democratic incumbent Bill Nelson. Some 35 Senate seats were in play, as were nearly 40 governorships and the balance of power in virtually every state legislature.
Several districts on the East Coast with early poll closing times were among those watched Tuesday for signs of the electorate's mood.
In Virginia's 7th District, a Republican-leaning area near Richmond, Republican Rep. Dave Brat was in a tight race with Democrat Abigail Spanberger, a former Central Intelligence Agency officer. Like other Democrats across the country, Spanberger emphasized protecting people with pre-existing conditions from being denied coverage or charged more by insurers. Now she faces a challenge from gun control activist Lucy McBath, whose son was shot and killed in 2012 after a dispute over loud music.
Sanders, the independent who has always been one of the state's most popular politicians, spent little time campaigning ahead of Tuesday's election. Harris was facing a stiff challenge from Marine veteran and small-businessman Dan McCready.
The tax law has been particularly problematic for Republicans in New Jersey, where four of five GOP-held seats were being seriously contested. McAdams believes that Democrats failed to offer an "alternative narrative" and that might hurt them at the ballot box.
"Senate Democrats faced the most hard political map in 60 years", said Sen.
About a half-hour later, he gave GOP Rep. Peter King a shout-out.
The GOP's gains come even as the results in Nevada and Arizona have yet to be determined.
There was a strong chance Mississippi's special election to complete the unexpired term of retired Republican Sen.
In Florida, six House seats are in play, and there are close and crucial battles for a Senate seat and the governorship.
"Speaker Ryan called the President [on Sunday] and said, "look, we had a great jobs report, let's talk about the economy" and so you have a lot of Republicans who want to talk about the record unemployment numbers that are out right now - this is one of the bright spots for Republicans, they want to talk this up, they think if they're going to be able to get suburban swing voters, that it's going to be the economy that brings these people to vote for Republicans, not this divisive immigration rhetoric".