A White House that has struggled to stay on course under favourable circumstances would be tested in dramatic ways.
Republicans are increasingly bullish that they will retain control of the Senate, but they face Democratic headwinds in the House.
Steyer insisted most Democrats agree.
On the eve of the election, Republicans edging ahead of Democrats by one point in a generic congressional ballot survey conducted by Rasmussen Reports, the polling company that came closest to predicting the outcome of the 2016 election.
"In view of analysts at ING, Tuesday's USA mid-term elections will have a major say over where the dollar trades for the rest of the year". He told the AP last month that he won't bear any responsibility should Democrats take over.
We certainly don't know what's going to happen. That is up 67.8 per cent from the 20.5 million early votes cast in all of 2014, the last federal election when the White House was not at stake.
Asked what happens if Democrats take the House, Trump was low-key.More news: Fox, NBC and Facebook turn down Trump ad deemed racist
Control of the House comes with subpoena power and the ability to use it to get his returns and potentially make them public.
The analysts also wrote that a potential error in the polls might not necessarily skew toward Republicans, allowing for the possibility of a bigger Democratic win than is now projected. First among them would be a potentially bitter leadership fight in the House to replace retiring Speaker Paul Ryan. While both Gillum and Abrams are African-Americans, the right-wing demonization of minorities and immigrants extends even to American Jews, not withstanding Trump's support for Israel on the foreign policy front.
"Clearly there's an terrible lot on the line in terms of the legislative agenda", said Republican consultant Josh Holmes.
Some in the White House think losing to Democrats might actually be preferable.
Laying into the tangled legal scandals enveloping the Trump administration - especially the possible collusion between his presidential campaign and Russian operatives - Mr Obama scoffed: "They've racked up enough indictments to fill a football team". They view House Minority Leader Nancy Pelosi as a potent foil for Trump, and believe they can tag the party responsibility for Washington dysfunction.
"No, he will have lost the House", Wallace said. He'd have to compromise with Democrats to get any legislation across the line.
But pro-Trump Republicans are eager to defy expectations, just as the president did with his 2016 victory.
"Even if I'm registering people who are going to vote for the other party, they're seeing us out here in our hats, and that's what's important, all the little interactions and conversations we're having", Kolkovich said.More news: Theresa May’s ‘secret Brexit deal’ with EU
That environment still might not be good enough to deliver the Senate, though. From stoking fears about illegal immigration to warning of economic collapse if Democrats are victorious.
And on Monday, he made the case that if Democrats win, they will work to roll back everything he's tried to accomplish.
"I think the President would personally attribute a lot of the Republican success to a validation of him", Ms Persons said.
"If you want more caravans and you want more crime, vote Democrat".
But Democrats are willing to wrestle with the political pressures and perils of impeachment because they know it's a problem they'll only encounter if they win the majority again.
Underlining the existential nature of the midterm contest, Mr Obama has also raised the ante, painting the election as a choice between two different Americas: one inclusive and embracing diversity, and the other narrow and exclusionary, and favouring corporate interests at the expense of ordinary Americans.
The president, who himself is not up for re-election until 2020, has been intensively campaigning in past weeks on behalf of Republican candidates in an effort to preserve his party's control of Congress.
At a recent rally, he called attention to his predecessor Barack Obama's middle name, which is Hussein - pandering to a long-running trope among conspiracy-minded conservatives that Mr Obama is secretly a Muslim.More news: Apple cancels iPhone XR production boost