(Reuters) - The battle for U.S. Senate control is among the biggest electoral fights in 2020, along with the presidential contest.

FILE PHOTO: U.S. Senator Doug Jones (D-AL) arrives for a briefing on the corona virus outbreak in China, on Capitol Hill in Washington, U.S., January 24, 2020. REUTERS/Joshua Roberts

Democrats need a net gain of four seats to win the majority in the Nov. 3 elections - or three, if presumptive presidential nominee Joe Biden defeats Republican President Donald Trump and gives Democrats the tiebreaking vote.

Here are the 10 most competitive races, according to independent analysts:

ALABAMA

Democrat Doug Jones is probably the most vulnerable senator, given Alabama’s deep Republican lean. His vote to convict Trump at his impeachment trial could hurt him in a state Trump won by nearly 30 percentage points in 2016.

Jeff Sessions, the former U.S. attorney general who previously held Jones’ seat, and Tommy Tuberville, the former head football coach for Auburn University who has gotten Trump’s endorsement, are vying for the Republican nomination.

COLORADO

Republican Senator Cory Gardner faces an uphill battle to win a second term in a state Trump lost by five percentage points in 2016. In 2018, Democrats swept statewide races, took full control of the legislature and picked up a U.S. House seat.

Gardner faces a top Democratic recruit: John Hickenlooper, the former two-term governor who ran unsuccessfully for president. Recent polls give Hickenlooper the edge, and he outraised Gardner $4.1 million to $2.5 million in the first quarter.

ARIZONA

Republican Martha McSally, a former Air Force combat pilot, was appointed to fill a vacant Senate seat shortly after losing her 2018 bid for the state’s other Senate seat. She faces Democrat Mark Kelly, a former astronaut and the husband of former U.S. Representative Gabby Giffords, who is now a prominent gun safety advocate after nearly dying in a mass shooting.

Kelly raised $11 million in the first quarter, nearly double McSally’s total. The state narrowly voted for Trump in 2016 and is seen as a toss-up in this year’s White House and Senate contests.

MAINE

Republican Senator Susan Collins has fended off challenges before, even though Maine has voted Democratic in every presidential election since 1992. But her 2018 vote to confirm U.S. Supreme Court Justice Brett Kavanaugh and to acquit Trump at his impeachment trial earlier this year damaged her image as a moderate willing to buck her party.

Her likely Democratic opponent, Maine House Speaker Sara Gideon, raised $7.1 million in the first quarter, while Collins brought in $2.4 million. Polls show a race too close to call.

NORTH CAROLINA

Republican Thom Tillis, a first-term senator, faces Army veteran and former state Senator Cal Cunningham in a toss-up race in this presidential swing state. Cunningham raised $4.4 million in the first three months of the year, more than double Tillis’ total.

MONTANA

Two-term Governor Steve Bullock, who abandoned his presidential bid in December, entered the Senate race in March after a sustained recruitment effort from national Democrats. He will take on first-term Republican incumbent Steve Daines.

Bullock has demonstrated the ability to win statewide despite Montana’s Republican lean - in 2016, he won by four percentage points while Trump notched a 20-point victory. But Trump’s popularity in the state could help carry Daines to another term.

IOWA

Republican Senator Joni Ernst is favored in a state Trump won by nine percentage points, but both parties see the race as competitive. The top Democratic challenger, businesswoman Theresa Greenfield, nearly matched Ernst in fundraising in the first quarter.

MICHIGAN

Democratic Senator Gary Peters will try to hold his seat amid a pitched battle between Biden and Trump in the presidential contest. Trump’s surprise Michigan win in 2016 proved crucial to his overall victory.

The presumptive Republican nominee, John James, is an Iraq War veteran who ran unsuccessfully for Senate in 2018. James has outraised Peters since last year, but the incumbent remains a slight favorite.

GEORGIA

Republican businesswoman Kelly Loeffler, appointed to the Senate in January, has seen her standing hurt by accusations of profiting from the coronavirus outbreak after financial disclosures showed she and her husband traded millions of dollars in stock just before markets crashed.

Loeffler has said outside advisers made the trades without her knowledge. Her challengers include Republican U.S. Representative Doug Collins, a strident Trump ally, and Democrat Raphael Warnock, a reverend. The candidates compete in an all-party election in November, with a January runoff if no one reaches 50%.

KANSAS

Kansas - where Trump won by 20 percentage points in 2016 - would not normally have a competitive Senate race. But Republican Senator Pat Roberts is retiring after 24 years in office, and U.S. Secretary of State Mike Pompeo, a former Kansas congressman, declined to run, leaving a crowded Republican nominating battle.

Democrats hope former Kansas Secretary of State Kris Kobach, who lost the gubernatorial election in 2018, emerges from the field. They believe his hard-right stances on voter ID laws and immigration could help the top Democratic challenger, former Republican state lawmaker Barbara Bollier.