But to pick up at least four Senate seats, Democrats probably have to win all four toss-up races. That, plus a win by Biden, would give them an effective Senate majority, since his vice president could cast tie-breaking votes.

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With a lot more unknowns than normal at this point in the election cycle, here are the 10 races most likely to flip. Because so many races are so close, rather than rank them, I grouped them into three tiers: Likely to flip; Toss-ups; Could flip under the right conditions.

Likely to flip: Alabama

Alabama (Democratic-held): Sen. Doug Jones (D) remains the most vulnerable senator in 2020. He won in 2017 against one of the most fundamentally flawed Senate candidates in recent memory, Roy Moore. And he won’t get that lucky again since Moore tried but did not make the Republican runoff. Jones will go up against either former attorney general and senator Jeff Sessions or former Auburn University football coach Tommy Tuberville. The one thing Jones may have going for him is that Sessions and Tuberville are locked in an expensive battle to win the nomination and may not have a ton of time to raise more money to mount a challenge against Jones in November. But do they need to? This is one of the most pro-Trump states in the nation, and Jones voted to convict Trump on both impeachment counts.

Toss-ups: Colorado, Arizona, North Carolina and Maine

Colorado (Republican-held): Democrats put themselves in a good position to unseat Sen. Cory Gardner (R) by convincing former governor John Hickenlooper (D) to run after his failed 2020 presidential bid. (Though he has yet to officially win the primary, Hickenlooper is the dominant candidate.) Colorado is rapidly trending left as Democrats hold the governor’s office and boost their voter registration thanks to younger voters moving to the state. Another bad sign for Gardner is that Hickenlooper outraised Gardner in the first three months of 2020. Gardner has hitched himself to Trump, refusing to criticize the president or say much of anything during the impeachment trial. Will keeping his base intact be enough for him in a state that voted for Hillary Clinton?

Arizona (Republican-held): Of all the Democratic challengers raising big money, gun-control advocate and former astronaut Mark Kelly is by far the most prolific. He has consistently outraised Sen. Martha McSally (R), who was appointed to the seat in 2019 after losing another Senate race in 2018. Kelly actually has nearly double the cash in the bank that McSally has, which is rare for a challenger. In addition, Arizona could be a presidential swing state (it narrowly voted for Trump and has a sizable group of voters of color).

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North Carolina (Republican-held): The growing political consensus on both sides is that North Carolina is the swingiest of swing states at all levels in 2020. Trump won by less than four points in 2016, as Democrats flipped the governor’s mansion and have since made big gains in the legislature. Now Democrats are trying to unseat Sen. Thom Tillis (R), who narrowly won his first term six years ago. Former state senator and Iraq and Afghanistan war veteran Cal Cunningham is the Democrats’ nominee (despite Republican interference to boost another candidate). In the first three months of this year, he doubled Tillis’s fundraising hauls. Tillis has tried at points to display some independence from Trump, pushing a bill to prevent the firing of Robert S. Mueller III and opposing Trump’s method of paying for the border wall. But he embraced the wall and during impeachment, he was all-in for the president.

North Carolina Democrats question whether that will backfire on Tillis if Trump gets poor marks for his coronavirus response. An early March NBC/Marist poll shows Cunningham up by several points. But with both parties and their respective outside groups likely to spend tens of millions of dollars here — and the race probably heavily reliant on the presidential fight — this is a complete toss-up.

4. Maine (Republican-held): Longtime senator Susan Collins (R) is one of the toughest Republican incumbents in this toss-up category for Democrats to knock off, since she’s such a well-known figure and voters have been sending her to the Senate for the past 20 years. But her tie-breaking vote to put Brett M. Kavanaugh on the Supreme Court was the catalyst for her top Democratic challenger, Maine House Speaker Sarah Gideon, to enter the race. Then Collins voted to acquit Trump in his impeachment trial. Now Collins must try to separate herself from the Trump administration’s slow response to the coronavirus. This race got even more competitive after Gideon reported what one Democratic strategist described as “a monster quarter” of fundraising, $7.1 million to Collins’s $2.4 million. But Collins is still a force in Maine. A February Colby College poll had the two in a dead heat.

Could flip under the right conditions: Michigan, Iowa, Montana, Kansas and Georgia special election

Michigan (Democratic-held): Michigan is one of the most hotly contested states in the presidential race, and the reelection bid of Sen. Gary Peters (D) will get caught up in that. Democrats say the fact that the coronavirus has hit Michigan hard makes it more likely Biden can win this state, which was crucial to Trump’s 2016 victory. In the Senate race, Republicans have made a big deal out of John James, an Iraq War veteran and conservative media darling. James has outraised Peters for three straight quarters and is close to having as much money as Peters in the bank. Democrats argue Republicans are too bullish on a candidate who also lost a Senate race against a Democrat in 2018. Polls have shown this race close between the two.

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Iowa (Republican-held): One GOP strategist says Sen. Joni Ernst (R) could face a more competitive campaign than any other race in this category, but Republicans argue this isn’t really a toss-up yet. For one, Iowa just isn’t a top swing state in the presidential race. Trump won it by nine points in 2016. Des Moines real estate executive Theresa Greenfield, the Democrats’ preferred candidate, came close to Ernst in fundraising in this past quarter, but she isn’t a well-known name in the state. Iowa Democrats, who have picked up two House seats since 2016, argue Ernst is more partisan than the state as a whole. She was vocal about supporting Trump during his impeachment, for example. A March Des Moines Register poll shows Ernst’s approval rating is down 10 points from last year, from 57 percent to 47 percent.

Georgia special election (Republican-held): Longtime incumbent Johnny Isakson retired from the Senate, which means both Senate seats in Georgia are up for election in November. (Sen. David Perdue (R-Ga.) was on our top 10 but has been moved off.) Sen. Kelly Loeffler (R) was appointed to Isakson’s old seat, but she’ll have to keep it by winning an all-party election in November that will go to a runoff in January if no one gets above 50 percent. Loeffler (R) is in hot water over stock trades made after a Senate briefing about coronavirus. (She says the trades were made by an independent third party.) Rep. Douglas A. Collins (R), who played a big role defending Trump during impeachment, is also running for the seat. Democrats hope a few things go their way: Biden does well in this state, the two Republican candidates drag each other down, and Democratic-leaning voters solidify around Atlanta pastor Raphael Warnock, who outraised them both recently. But Warnock has yet to persuade other Democratic candidates to drop out. His path to victory probably depends on getting over 50 percent in November, which for now is an uphill climb.

Montana (Republican-held): Can a popular Democratic governor who won in Trump country unseat a sitting Republican senator? Term-limited Gov. Steve Bullock (D), a former 2020 presidential candidate, is running against Sen. Steve Daines (R). Bullock is the Democrat with the best shot, given he’s won three times statewide, including when Trump swept the state in 2016. And in 2018, Sen. Jon Tester (D) won a tough reelection fight. But can Bullock unseat a sitting Republican senator in a state that some strategists estimate could vote for Trump by as many as 20 points?