Trump's ability to pass any major legislation in the next two years — at the very least. The Russia investigation, too. At the most extreme? Justice Brett Kavanaugh's standing and, if the most militant Democrats get their way, an attempt to impeach Trump.
More broadly, this midterm has shaped up as a test of the stunning results of 2016. Was it a freakish outcome, spurred on by an extraordinary mix of events? Or an inflection point, when Trump shattered the old rules?
This election won't provide definitive answers, but it is the first national measure since 2016.
If Democrats take one or both chambers of Congress they'll have the power to block bills for at least the next two years, much as Republicans used their House majority to stifle Barack Obama's biggest initiatives after the 2010 midterm elections.
They'll also gain the power to lead committees and conduct investigations — and have promised loads of them.
While House Republicans have tried to shield Trump from the special counsel probe into Russian election interference, Democrats have pledged to drill deeper. They'll be able to subpoena key figures and fight GOP efforts to undermine investigators.
Democrats have also pledged to look into potential self-enrichment within the administration, and could put more scrutiny on explosive issues such as the work to reunite children and parents separated at the southern border. Some Democrats want to dig into whether Kavanaugh lied under oath in his Senate nomination hearings. At the far end of the spectrum, others have broached impeachment of Kavanaugh, Trump, or both, though most party leaders have urged caution.
A Democratic House alone, however, would not be able to stop Trump from appointing more Supreme Court justices, should additional vacancies arise. And any impeachment efforts would almost certainly run aground in the Senate, even if Democrats win that chamber (against much tougher odds).
Democratic control in part of Congress could also provide Trump a useful foil. He would have a convenient target should the economy stumble or other problems arise. Democrats, now limited to opposition, would have some responsibility for legislating, keeping the government running, and laying out their ideas, rather than just saying "no" to the White House.
Still, they'd certainly prefer that option to the status quo.
As for Trump, the election might not say much about 2020.. Obama and Bill Clinton both lost big in their first midterms, and still won second terms.
It's hard to think of a race in which Trump isn't a factor.
For the past three years, all of American politics has essentially boiled down to the question: Are you with him? Or not?
Voter enthusiasm was at the highest level for any midterm in two decades, according to a Pew Research Center poll in late September, and 60 percent of voters viewed their decision as an expression of either support for Trump or opposition to him.
And that was before the Kavanaugh Supreme Court fight inflamed voters on both sides. Still, in some races, the president's influence is particularly profound.
Pennsylvania's First Congressional District: Some Republicans, like U.S. Rep. Brian Fitzpatrick in the Bucks County district, are trying to localize their races, and many Democrats are avoiding talking explicitly about the president, too — because they say his Twitter feed says it all. Fitzpatrick has an independent reputation and an established name, since his brother represented the district for four terms. His race will hint at whether Republicans can maintain a distinct identity, or if the president's influence overwhelms everything.
Pennsylvania governor and U.S. Senate in Pennsylvania: The top of the Republican ticket in Pennsylvania reflects how the GOP has embraced Trump, and makes the state a test of whether his approach transfers to other candidates. Scott Wagner, the nominee for governor, is a brash businessman who is unafraid to offend. U.S. Rep. Lou Barletta has played up his blue collar appeal and hard line on illegal immigration.
New Jersey's Third Congressional District: U.S. Rep. Tom MacArthur, a Republican, backed Trump's tax bill and attempted roll back of the Affordable Care Act. He was the only House member in suburban Philadelphia to support both plans.
What hasn't happened?
Most recently Trump landed Kavanaugh on the Supreme Court, sealing a conservative majority for years to come. The result enraged Democrats, but may also fire up Republicans who felt the nominee was mistreated.
Good economic news should benefit Republicans — though that is often overshadowed by Trump's latest firestorm.
Trump, and his Twitter feed, remain the biggest variables of all: what will he cook up in the days before the vote? At this moment, even he probably doesn't know.