The House majority will be determined in dozens of suburban districts across the country, while the future of the Senate is up for grabs in Republican-leaning states where Trump won in 2016 and his support is solid among rural voters.
In those suburban districts, the question is whether swing voters go with their wallets - the months of positive economic news of job and wage growth - or concerns about their healthcare.
"A booming economy or the radical policies of the liberal mob - that's our choice," says one GOP ad running on behalf of Congressman Steve Chabot, (R), who represents the Cincinnati area.
In the Chicago area, Democrats are blasting Congressman Peter Roskam, (R), with an ad showing a child in a hospital bed: "Imagine watching him going without lifesaving treatment because it's been denied by your insurance. That's what Peter Roskam voted for."
House Minority Leader Nancy Pelosi said healthcare has been paramount, reflecting a concerted Democratic effort that started in the days after Trump's election to preserve the ACA from Republican attempts to scrap or gut the law.
"It's dominant because it's dominant in the well-being of people's lives," she said. "It's also dominant because we made it so."
A Washington Post-ABC News national poll published today found Democrats with a seven-point national advantage among likely Midterm voters, which is in the range forecasters predict is necessary to flip control of the House. The lead is driven by an educational divide: Those with college degrees heavily favour Democrats.
The picture is more favourable for Republicans in the Senate, where Democrats are defending 10 seats in states Trump won. Many of those states are predominantly white and lean Republican, and Trump's immigrant comments resonate.
John Rogers, executive director of the National Republican Congressional Committee (NRCC), predicted that the threat of a Democratic "resistance mob mentality" that would impede the Trump agenda, as well as policy proposals supported by some on the left, will swing persuadable voters and save the GOP's eight-year House majority.
"I think the voters are out there seeing all this and saying, 'Maybe I want a check and balance, but this is not what I want. This isn't the check and balance,' " Rogers said.
Trump repeatedly casts Democrats as an angry mob while ratcheting up his false claims about the party wanting to allow millions of illegal immigrants into the country, including the thousands who are part of a migrant caravan slowly heading toward the border.
He also has proposed to end birthright citizenship for the children of illegal immigrants.
Other Republicans have joined Trump in seeking to energise the base with dark images of immigrants and references to the caravan.
In Tennessee, GOP Congresswoman Marsha Blackburn is airing images of the caravan alongside video of Democratic opponent Phil Bredesen saying the migrants are "not a threat to our security".
Trump's "doing that because they have nothing else to talk about," said Dan Sena, director of the Democratic Congressional Campaign Committee (DCCC). "The country does not support their tax package, and the country does not trust them on healthcare."
In Nevada, a Democratic super PAC is bombarding Senator Dean Heller, (R), with a spot saying he "puts his political party over your healthcare" and warning that GOP leaders are eyeing Medicare cuts.
Late TV spending for the DCCC and the House Majority PAC, the biggest Democratic groups focused on House races, are overwhelmingly promoting messages targeting Republican healthcare policy and last year's GOP tax bill, which Democrats are arguing will prompt cuts to Social Security and Medicare.
In New York's 22nd Congressional District, Democratic ads have been hammering GOP Congressman John Faso for months, highlighting video of him promising to protect health coverage for people with pre-existing conditions.
"John Faso broke his promise," a DCCC ad says. "It's time for him to go."
A key figure in the GOP messaging continues to be Pelosi, who is seeking to become speaker if Democrats win a majority.
Of 16 ads debuted in the past week by the NRCC, 10 mention Pelosi or feature her image. An additional half-dozen spots aired by a major GOP super PAC, the Congressional Leadership Fund, also feature Pelosi.
"Your vote decides if Pelosi becomes speaker again," says one NRCC ad running against candidate Elissa Slotkin of Michigan, calling a vote for Slotkin "a vote for Pelosi agenda - for tax hikes, for amnesty, for chaos."
Democrats are waging a perennial struggle to reach Democratic voters who tend to vote in presidential elections but not Midterms - many of them African American, Hispanic and young voters.
The group launched a US$30 million engagement campaign specifically targeting those groups - an effort that began days after Trump's win in 2016.
Early-voting data, Sena said, indicated that the Hispanic share of the overall vote was more in line with the 2016 presidential year than the 2014 Midterm year.
That kind of Latino turnout, he said, "really paves the way for us to win the House in multiple different ways".
The DCCC's national advertising efforts include multistate ads running on African American radio stations and on Spanish-language television.
Those ads, Sena said, feature a positive message aimed at inspiring, rather than scaring, voters.
Rogers, the NRCC leader, acknowledged that the immigration issue has been "largely base-motivational," but said that his group also has been focused on persuading married college-educated women, a demographic that has turned sharply away from Trump.
"The voters don't have one No. 1 issue that is driving this election," he said. "I think it's what's caused this to be a very volatile election cycle all the way to the end, which is where we are now, and I think these good economic numbers are going to be really good for us going in the close here."
The widely seen bounce in GOP voter energy around the confirmation of Supreme Court Justice Brett Kavanaugh earlier this month might have faded somewhat, Rogers said, but it has improved the party's fortunes in key races.
"We're now back into fighting out each of these individual races on their own terms," Rogers said.
