WASHINGTON — The 2020 battle for control of the Senate gets underway in earnest Tuesday, with both parties increasingly seeing the chamber as a must-win prize to guard against uncertainty over who will wind up in the White House.

Republicans, seeing little chance of reclaiming the House and facing prospective Senate losses, are working to hold their majority to maintain their alliance with President Donald Trump should he win reelection or serve as a firewall against a Democrat in the Oval Office. Democrats, who detect some opportunities against Republican incumbents struggling with home-state voters, regard control of the Senate as their only hope of reining in Trump if he wins, and a crucial perch to support a new presidential ally should Trump fall short.

“I always thought that as we got closer to the election, more and more people would focus on the Senate, and that is what is happening,” Sen. Chuck Schumer of New York, the Democratic leader, said in an interview.

With the vice president empowered to cast tiebreaking votes in the Senate, Democrats need a net gain of three seats to take the majority if the party captures the White House; four if not. Republicans can afford to lose up to three seats and hold their majority as long as Trump is reelected. Both sides acknowledge the majority is up for grabs in a chamber paralyzed by dysfunction and intense partisanship, whose reputation for independence and reasoned debate has been tarnished. A photo finish is not out of the question.

“The Senate majority is in play,” said Nathan L. Gonzales, a nonpartisan handicapper and editor of Inside Elections. “The races are close enough that a 50-50 Senate is a real possibility.”

Voters in Alabama, North Carolina and Texas will make the first crucial Senate candidate selections on Super Tuesday. The environment is volatile, unsettled by the bitter Democratic presidential primary, the potential fallout from the failed effort to remove Trump from office, and now the rising threat of the coronavirus and its economic and health implications. Some Democrats in Congress have begun to sound the alarm about potential down-ballot damage if Sen. Bernie Sanders, I-Vt. and self-described democratic socialist, is the party’s presidential nominee.

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But Republicans have news for Democratic contenders for the Senate: They will be branded socialists whether Sanders is at the top of the ticket or not.

“Bernie will be on the ballot regardless, because every Democrat has gone there and we will spend whatever it takes to make sure voters knows it,” said Kevin McLaughlin, the executive director of the National Republican Senatorial Committee, referring to expansive social programs embraced by Democratic presidential candidates during the raucous primary campaign.

The strategy is already apparent. In Arizona, Sen. Martha McSally, a Republican who has trailed in polls against her Democratic challenger Mark Kelly, a former astronaut, hit Kelly as “too liberal” for the state in a recent ad entitled “Bernie Bro.” In Georgia, Sen. Kelly Loeffler, a newly appointed Republican who will be on the ballot in November, declared in an ad that “socialism risks everything that makes us great.”

Sen. Mitch McConnell, R-Ky., the Senate majority leader, underscored the coming approach in his own comments last week. “If you look at what the various candidates for president on the Democratic ticket are saying, there’s not a whole lot of difference between any of them,” he told reporters. “They all look pretty much the same to me, and pretty much the same is very, very far to the left.”

Democrats say they are ready to counter that argument and have carefully selected candidates who have carved out their own distinct political ground to match their state. Most Senate Democratic challengers have already disavowed “Medicare for All,” a centerpiece of Sanders’ agenda, saying instead that Congress should focus on improving the Affordable Care Act.

“Our candidates chose to run independent of what was happening at the top of the ticket,” Schumer said.

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The first test for Democrats and how well they have gauged the field will be on Tuesday, when all eyes are on the presidential race but voters will make pivotal decisions that will shape the race for control of Congress. In North Carolina, where Democrats hope to defeat Sen. Thom Tillis, a Republican who polls show is strikingly unpopular, Senate Democrats are supporting Cal Cunningham, a military veteran and former state lawmaker, over the state senator Erica D. Smith. Smith received unusual advertising support during the primary from a group with ties to McConnell in a Republican effort to help pick Tillis’ opponent.

A loss by Cunningham would be a setback for national Democrats who see him as the strongest challenger to Tillis, a top target. In another Tuesday primary, Senate Democrats have endorsed M.J. Hegar, a former Air Force helicopter pilot who is running in a crowded field in Texas to take on the Republican senator John Cornyn.

Republicans have a high-profile primary of their own on Tuesday in Alabama, where the incumbent Doug Jones is the most endangered Senate Democrat to face reelection this year. The contest is among Jeff Sessions, the former senator trying to regain his seat after a rocky tenure as Trump’s attorney general; Tommy Tuberville, a former football coach of Auburn University; and Rep. Bradley Byrne. With Sessions and Tuberville the favorites in recent polling, it is likely headed to a runoff on March 31.

At the moment, Republicans consider their biggest problem to be a nasty intraparty fight in Georgia, where Loeffler is being challenged by Rep. Doug Collins, the top Republican on the Judiciary Committee who was a steadfast defender of Trump during the impeachment inquiry. Despite intense pressure from Senate Republican leadership — and a hint from Trump that he could be named national intelligence director — Collins has refused to bow out, and the two Republicans are in an escalating feud.

Since it is a special election to fill a vacancy, candidates from all parties will be on the Nov. 3 ballot, and Republican strategists fear that the internal battle will allow Democrats to slip in and take control of the race — even potentially imperiling Georgia’s other Senate seat held by the Republican David Perdue, who is facing reelection.

Republicans also have concerns in Kansas, where McConnell tried unsuccessfully to entice Secretary of State Mike Pompeo into running. Now another well-known but polarizing candidate, Kris Kobach, who lost a run for governor in 2018 to a Democrat, is leading in the polls. Republicans are concerned Kobach could cost them a seat in what should be a reliably Republican state. Democrats see as formidable their preferred candidate, Barbara Bollier, a state senator who left the Republican Party to become a Democrat in 2018.

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Democrats and their allies are already pouring tens of millions of dollars into defeating the Republican incumbents Cory Gardner in Colorado, Susan Collins in Maine, Joni Ernst in Iowa, Tillis and McSally while taking a run at seats in Georgia, Kansas and perhaps Texas. Republicans hope to take back the Alabama seat, protect their incumbents and try to steal another seat away from Democrats, with Michigan — now represented by Sen. Gary Peters — considered their best prospect.

Adding to the uncertainty, Georgia has set a date of Jan. 5 for a runoff should no candidate in the special election there win a majority, meaning that control of the Senate could be in doubt into early next year.

Republicans note that even if they lost a few incumbents, they could retain a narrow majority. Democrats believe they can seize the majority irrespective of who wins the presidency.

Electoral patterns over the last decade have shown that presidential and Senate voting is much more closely correlated than it had been in the past, with fewer ticket splitters. Both parties may need to defy that trend if they hope to end up controlling the Senate gavel in 2021.