There’s a good chance Joe Biden will defeat President Trump next week. Whether he’ll be able to do anything as president is another question—one that hinges on how much damage Trump does to down-ticket Republicans. The difference between a narrow Biden win and a comfortable victory will likely determine whether he’ll have a Democratic Senate to work with. Strategists in both parties say Democrats have a slight edge in the battle for Senate control, though no one thinks the fight is over. And the path to the Senate majority runs mostly through red-tilting states where Biden currently is tied or has a slight lead. Biden’s travel seems to suggest he’s looking not just to win but also to bring a Democratic Senate majority with him. He’s been to Arizona, North Carolina, and Iowa in recent weeks—all states he can lose but still win the presidency—and stopped in Georgia, which has two crucial Senate races, on Tuesday. ”Folks, I think we’re going to surprise the living devil out of everybody this year. I can’t tell you how important it is that we flip the United States Senate. There’s no state more consequential than Georgia in that fight,” Biden said during an Atlanta campaign stop Tuesday evening. “If we told all these guys out on the street a year ago how competitive Georgia was going to be, you all would have looked at one another like, ‘You’re crazy.” That’s pretty much true. While Democrats had long fantasized about putting Georgia in play, the commonly held assumption was that Democrats would likely lose the state at the presidential level, and they’d need a candidate who could run ahead of their White House nominee to win Senate seats there and in other GOP-tilting states to have a shot at Senate control. Now it looks like Biden just might win these states—and provide enough coattails to win the Senate. Republicans are worried about Trump’s standing in all of these states—and what it might do to their Senate majority. “His numbers look like shit in North Carolina, Arizona, and Georgia,” said one GOP strategist who has seen a lot of data in the three states. “The question is whether the polls are off again.” One GOP strategist involved in a number of Senate races said if Biden wins those states and Iowa, “at that point then, you can start saying goodbye to senators; the wave comes and gets everybody.” If Biden wins, Democrats need to pick up a net of three seats for a 50-50 tie that would give them control. More than a dozen strategists involved in Senate races from both parties involved in Senate races generally agree that Democrats are slight favorites to win the Senate—but say how Biden performs and whether the polls are right will determine whether Democrats take unified control of Congress and the White House for the first time since Biden was elected with President Obama in 2008. They’re almost certain to lose Democratic Sen. Doug Jones’ Alabama seat, and almost as likely to defeat Colorado Sen. Cory Gardner. While strategists in both parties say Arizona is closer than public polling would suggest, Sen. Martha McSally has run consistently behind Trump in a state where Biden is slightly ahead in most public and private surveys, and the consensus is that former astronaut Mark Kelly will win the seat for Democrats. Sen. Susan Collins is basically tied with Democrat Sara Gideon in private head-to-head polling in Maine, but Republicans concede that the state’s ranked-choice voting, where votes for third-party candidates get reallocated to their second choice, makes Collins the slight underdog. That leaves Democrats one seat shy of a majority, with a pair of GOP senators in states with close presidential contests facing tough reelections that will likely determine Senate control. North Carolina Sen. Thom Tillis got a lifeline when his opponent, Cal Cunningham, recently got caught having an extramarital affair. But GOP strategists say while that hurt Cunningham’s favorable numbers, it didn’t budge the race, which is tracking very closely to the close presidential election in the state. Tillis still trails Cunningham by a bit, and unless the polls are wrong or the race shifts in the final week, both sides think Tillis will lose. “What happens in the presidential will be what happens down-ballot. Cal’s scandal has eroded his image. The problem is he still has a better image than Thom Tillis,” said one Senate GOP strategist. Iowa is another state where the Senate race is closely connected to the presidential results. Public and private polling shows GOP Sen. Joni Ernst is in a dead heat with Democrat Theresa Greenfield. Both parties think they have a slight edge in the state. Democrats’ next-best opportunity is in Montana, where Democratic Gov. Steve Bullock has hung around against Sen. Steve Daines. Strategists in both parties think the race is statistically tied, though Republicans seem to feel a bit more confident than Democrats about winning the seat. Democrats are also increasingly bullish about Georgia, which has two Senate races this election. Sen. David Perdue is locked in a tight battle against Democrat Jon Ossoff, and if Biden carries the state, Democrats think Ossoff has a shot at an outright win on election night. Strategists in both parties think the most likely result is neither candidate reaches 50%, however, which would lead to a runoff election. The state’s other Senate seat is likely headed to a runoff as well. Democratic pastor Raphael Warnock will face either GOP Sen. Kelly Loeffler or Rep. Chris Collins in that race. Loeffler has run hard to the right to try to defeat the very conservative Collins, and Democrats think if the national environment stays strong for Democrats, Warnock could score a win. Georgia’s early January runoffs mean that the battle for the Senate could go to overtime if Democrats fall short. If Biden has won and Democrats already have Senate control, however, Democrats concede that it might be harder to motivate their base to turn out in high enough numbers to win either of these races. Republicans have also made noise about defeating Sen. Gary Peters in Michigan, another state Trump won last time, but they admit that if Trump can’t narrow his margins there, they’re likely to lose that race as well. Senate Majority Leader Mitch McConnell (R-Ky.) has repeatedly said Senate control is a 50-50 proposition, and he seemed even more pessimistic about the election on Sunday during his Senate floor remarks celebrating Judge Amy Coney Barrett’s pending confirmation to the Supreme Court. “We've made an important contribution to the future of this country,” he said . “A lot of what we’ve done over the last four years will be undone, sooner or later, by the next election. They won’t be able to do much about this for a long time to come.” Trump’s stabilization in the polls has likely put some other races beyond reach for Democrats. Republicans had been growing increasingly worried that a Trump collapse and their own candidates’ missteps could cost them a Kansas seat and endanger Sens. Lindsey Graham of South Carolina, Dan Sullivan of Alaska, and John Cornyn of Texas, but they feel pretty sure right now that all three seats will stay in their column. Democrats think these races are still close, with Kansas and South Carolina within a few points, and say Texas is trending their direction, but unless Biden has a sweeping victory, the GOP is likely to hang on in these contests. Senate races tend to break the same way on Election Day, with one party winning a majority of the tossup seats. Strategists in both parties see a plausible path for Republicans to hang onto 51 seats and Senate control — or for Democrats to get up to 53 seats or so, enough for a functional majority to support Biden’s agenda. But right now, Democrats who thought they’d be needing to outpace Biden to win the majority are hoping he can pull them across the finish line.