With less than a month until Election Day, Democrats are picking up momentum toward winning the Senate majority: Challengers are outraising Republican incumbents, and they are leading in polls. They’ve been helped along by Democrat Joe Biden’s widening lead over President Trump, and some Senate Republicans seeking to distance themselves at the last minute from Trump, after spending years backing him.

But anything could happen. Like this: the revelation of an affair involving the Democratic nominee in one of the most pivotal states, North Carolina, as many voters there are casting ballots. And an increase in mail voting carries with it the risk that voters who don’t fill out their ballots correctly won’t be able to vote at all.

Democrats need a net gain of at least four Senate seats to win the majority, or a Biden presidency and three net wins, which would give the vice president the deciding vote in any ties.

Democrats have a chance in a dozen of the 14 races on this list, but some are in solid Republican territory. We removed Senate Majority Leader Mitch McConnell’s (R-Ky.) reelection race from our rankings — he’s ahead of Democratic challenger Amy McGrath despite the tens of millions of dollars she’s raised to make it competitive.

Here are the top races most likely to flip parties, categorized and ranked from most to least likely.

More likely to flip than not: Alabama, Colorado and Arizona

1. Alabama (Democratic-held): There are no changes to the top of our list. Sen. Doug Jones (D-Ala.) remains the most vulnerable senator of 2020. He’s running for reelection in one of the most pro-Trump states in the nation. And unlike in 2017, he’s not going up against a seriously flawed Republican opponent. Former Auburn University football coach Tommy Tuberville has a clear path to victory here.

2. Colorado (Republican-held): Sen. Cory Gardner (R) is trying to pull away from Trump in this purple-blue state. He was hoping his Democratic opponent, former governor John Hickenlooper, would be weakened by an ethics scandal, but that wasn’t enough of an issue to overcome partisanship — and anti-Trump sentiment on the left. Said one Republican strategist: “If the president is really struggling in Colorado, that makes the math difficult for Cory.”

3. Arizona (Republican-held): Republicans had hoped this race was tightening for them, but polls show the opposite. A New York Times-Siena College poll in early October has Democratic challenger Mark Kelly up by an astounding 11 points over Sen. Martha McSally (R), who was appointed to the seat left vacant following John McCain’s death. Even if that poll is an outlier, Kelly has been consistently leading and outraising McSally. And Biden has led here by an average of three points in high-quality polls since September. Democrats could have historic wins here in November. In a sign of what trouble she’s in, McSally in a debate this week refused to say whether she was proud of her support for the president.

Toss-ups: Maine, North Carolina, Iowa, Georgia, South Carolina, Montana

4. Maine (Republican-held): The polls are just not breaking Sen. Susan Collins’s (R) way. Two high-quality polls in September had Democratic challenger, Maine House Speaker Sara Gideon (D), up by about five points. Maine was never a state Trump was expected to win, so Collins always needed to get the votes of some Biden supporters. But as Trump’s poll numbers sink, she will have to win over even more Biden voters to keep her job, and that just gets harder to do. Collins became the subject of Democrats’ ire after a 2018 Supreme Court battle. Now there’s another big one that Democrats are trying to leverage to further tie Collins to Trump. That’s likely why Collins wasted no time saying she opposes her party’s push to confirm a justice before the election.

5. North Carolina (Republican-held): This swing state at nearly all levels of government has also been trending Democrats’ way, including the Senate race. But what happens now that Democratic Senate candidate Cal Cunningham’s affair — complete with salacious texts — has been revealed weeks before Election Day? It’s too soon to tell. Republicans made a compelling argument when they said Cunningham single-handedly managed to undercut his central premise as a candidate about honor and duty as a veteran and family man. In a disastrous interview Friday with North Carolina journalists, he refused to answer multiple direct questions about whether there were other women. Now he’s under investigation by the Army Reserve for poor behavior. A University of Massachusetts Lowell poll taken before the affair was revealed showed Cunningham up six points, and Cunningham had announced he had raised an insane $28 million in the past three months, while Sen. Thom Tillis (R) had just contracted coronavirus after being at the White House. “We do see this as a big break in a race that needed one,” said a Republican strategist who agreed to speak on the condition of anonymity.

6. Iowa (Republican-held): Iowa still leans red, but Democrats now have a voter registration advantage over Republicans in the state. One Democrat joked that may be the only good thing to come out of the disastrous Iowa presidential caucuses in February. And as Senate Democrats’ campaign arm doubled its investment here to try to unseat Sen. Joni Ernst (R), a Quinnipiac University poll in early October had Democratic challenger Theresa Greenfield up by five points and hitting the all-important 50 percent mark. Greenfield also just announced she raised a record-breaking $28.7 million over the past three months. She’s helped by Biden having a two-point average lead here, based on high-quality polls tracked by The Washington Post. This race will likely be close until the end, and it could decide the majority.

7. Georgia (Republican-held): Democrats have two shots to try to unseat Sen. David Perdue (R): (1) in November, and then if neither candidate gets over 50 percent of the vote, (2) a runoff in early January. Like all the other races in this toss-up category, it’s tight. Democrat Jon Ossoff’s best chance to win would be to get 50 percent in November, a feat that would probably require Biden to win the state at the presidential level by several percentage points. High-quality polls show Biden is polling about even with Trump here.

8. South Carolina (Republican-held): That Sen. Lindsey O. Graham (R) is in a toss-up race is probably the Senate’s biggest surprise of 2020. Some Trump supporters are peeling away from Graham, despite his overt embrace of the president. Democrat Jaime Harrison has raised millions of dollars and flooded the airwaves with a personal story that is motivating Democratic-leaning voters in the suburbs — the early-voting lines are some of the longest South Carolina politicos have seen. Polls have regularly shown the race tied. A Republican super PAC just poured a precious $10 million to try to shore up Graham. And Republicans are hoping that Graham leading the Senate hearings for Supreme Court nominee Amy Coney Barrett will help him, too.

9. Montana (Republican-held): Trump won this state by 20 points four years ago, but a New York Times-Siena College poll last month found him with just a seven-point lead and the Senate race basically tied. Still, Republicans have evidence showing Sen. Steve Daines (R) regaining his footing in these final weeks against his challenger, the popular Gov. Steve Bullock (D), as Republicans try to smack him with negative ads. Montana voters are known for being okay voting for a Democrat and Republican on the same ballot — like they did in 2016 voting for Bullock for governor and Trump for president. But in a hyperpartisan presidential year, there just may not be enough reason for Montana voters to kick out their sitting Republican senator.

Could flip under the right conditions: Michigan, Georgia’s special election, Kansas, Alaska, Texas

10. Michigan (Democratic-held): Sen. Gary Peters (D) has had a sleepy race against Republican challenger John James, a veteran and conservative media darling. But that may be changing as outside groups pour millions of dollars into this race, causing polls to tighten in a way that worry some Democrats. As The Washington Post’s Rachael Bade and Paul Kane reported recently: “Some allies of Peters have sent warnings to Democratic leaders that if they do not shore up what should be an easy victory, they could fall shy of the majority.” Still, this is the most Democratic-friendly swing state at the presidential level, with Biden leading by an average of seven points according to a Washington Post average.

11. Georgia special election (Republican-held): Democrat Raphael Warnock has an infusion of cash that is helping him take the lead in some polling in this crowded special election race. Two other Republicans are duking it out — Sen. Kelly Loeffler and Rep. Douglas A. Collins — and it’s likely no one gets to 50 percent, sending the top-two vote getters to a runoff. If Warnock makes it, he’ll have to convince Democratic-leaning voters to come out again in January, when there won’t be a presidential race to help him overcome the conservative tilt of this state.

12. Kansas (Republican-held): Republicans aren’t happy with how their candidate, Rep. Roger Marshall, is doing against former-Republican-turned-Democrat state senator Barbara Bollier. She’s outraising and outspending Marshall as she tries to seize the health-care mantle, forcing GOP outside groups to spend millions of dollars here to make sure they don’t lose this race — a possibility after Democrats won the governor’s race here in 2018.

13. Alaska (Republican-held): Democrats are supporting independent Al Gross, a doctor and fisherman with money of his own to spend as he tries to unseat Sen. Dan Sullivan (R), who narrowly beat a well-known Democrat six years ago. Yet again, Trump’s handling of the coronavirus pandemic is the top reason it has the potential to be competitive, with polling showing Trump leading by single digits and some private polling showing Gross down by about four points. Alaska does have an independent streak and has elected Democrats to the Senate, but as a Republican strategist put it, this is a state that wants to vote Republican.

14. Texas (Republican-held): We debated taking Texas off this list after Sen. John Cornyn (R) strengthened his lead over Democrat MJ Hegar in several recent polls. But Hegar does currently have more ads on TV than he does, and Democrats argue that if the Latino community gets marshaled, she could be in a great position. Biden is also behind Trump by just about three points here, according to a Washington Post average of polls. Both sides agree Texas could go blue — but it will probably take a much more concerted (read: expensive) investment on Democrats’ part to do it. And even as Hegar raises millions of dollars, so far the outside money isn’t there.

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This has been updated with Cunningham’s latest comments about his affair.