The Trump administration now holds power in all branches of government - the presidency, the Congress and the Supreme Court - and if Democrats were to take back the House, they could put a pin in much of Trump's agenda.
South Florida voters cast their ballots at a polling center in Miami, Florida on November 6, 2018. In the Senate, Republicans are cautiously optimistic they will hold onto - or even add seats to - their slim 51-49 majority.
Democrats need to pick up two dozen seats to claim the House majority.
In the final stretch, Mr Trump hardened his rhetoric on issues that appealed to his conservative core supporters, issuing warnings about a caravan of Latin American migrants headed to the border with Mexico and condemnations of what he called United States liberal "mobs".
Trump flew to Fort Wayne just days after stumping in Indianapolis, where he brought along native son Vice President Mike Pence and pugnacious basketball coach Bobby Knight in an effort to unseat Donnelly.
At his rallies and on Twitter, Trump's closing argument has largely focused on fear - warning, without evidence, that a Democratic takeover would deliver the country into socialism, spurring an influx of illegal immigration and a wave of crime.
"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.
"People are coming out because they care about taking back the House and turning this country around", she said, calling Pelosi an "inspiring leader".
"Let me say this".
Last week, Democrats led by three points, 47% to 44%.
Fact-checker sites state that Trump got 41 percent of the female vote in that election, though he did get 52 percent of votes from white women. When you participate in the political process, you can be a check on bad behavior.
When Blackburn took the stage briefly, she told supporters, "If you want to vote no to Hillary Clinton and her cronies one more time, stand with me".
But anti-incumbent fervor helped sweep House Democrats out of power in 2010, during President Barack Obama's first term, and it threatened to hurt Republicans this year, dramatically remaking the map in key states from Pennsylvania to California. Donaldus Magnus is driving everything left and right, Republican and Democrat.
Tuesday's elections also tested the strength of a Trump-era political realignment defined by evolving divisions among voters by race, gender, and especially education.
"I think we're going to do well in the House", he said of Tuesday's races. At the same time, Republican governors in Maryland and MA - traditionally blue states - look poised for easy re-election victories. RealClearPolitics puts Trump's average approval at about 43 percent, with 53 percent disapproving.
The president has vowed to continue pushing for significant funding for a border wall, which Republicans may feel more comfortable with backing if voters hand them power again. "That is particularly true when a president is unpopular, as this president is". But two years after an election that proved polls and prognosticators wrong, nothing is certain on the eve of the first nationwide elections of the Trump presidency.
"The reasonable range of outcomes in the Senate still seems fairly wide, with a bigger GOP gain possible, or no gain at all or even a Democratic gain", they explained.
University of Miami graduate student Maddie Kaufman, 24, said she was voting Democrat because of climate change.
A Democratic majority in the House would nearly certainly bring an onslaught of investigations into Trump's businesses and his administration.
Instead of trumpeting the GOP's $1.5 trillion tax cuts, the debate was dominated by Trump's dire prediction of "invasion" from the migrant caravan and what he called the "radical" agenda of speaker-in-waiting Pelosi.
Amid the recent rash of letter bombs and the Pittsburgh synagogue massacre, Trump issued alarming and often unfounded warnings about caravans of migrants crossing Mexico toward the US, blaming Democrats, without evidence, for the threat he claimed they pose. "Anything that gets done will have to be a bipartisan basis".