WASHINGTON — With 1,344 pledged Democratic delegates up for grabs on Super Tuesday, most of the nation's attention will be focused on the battle for the party's presidential nomination.
But a Republican White House isn't the only thing on the line this November. Republican control of the Senate is also up in the air, with the GOP defending 23 Senate seats, while Democrats are defending only 12.
On Tuesday, the battle for three of those Senate seats will officially begin, in North Carolina, Alabama and Texas, where primary voters will pick challengers to run against those sitting senators in November.
Here's what's at stake in each race.
Incumbent Republican Sen. Thom Tillis has one of the lowest approval ratings of any GOP senator up for reelection, and Democrats consider his seat a prime pickup opportunity. Tuesday's primary is expected to answer the question of who will run against Tillis in November.
The powerful Washington-based Democratic Senatorial Campaign Committee, or DSCC, is backing former state Sen. Cal Cunningham, a military veteran who is considered the most moderate Democrat in the primary. His closest rival is state Sen. Erica Smith, who has staked out a more progressive platform.
An NBC/Marist poll released Sunday showed Cunningham winning support from 51% of likely Democratic primary voters, while Smith got 18%.
The race is also noteworthy because a group with ties to Republican Senate Majority Leader Mitch McConnell of Kentucky paid $1 million for ads promoting Erica Smith.
The voice in the ads calls Smith "one of us," strongly implying to viewers that the ads are being paid for by a Democratic group. In reality, they're a ploy by Republicans to boost the Democrat who they think Tillis would have the best chances of beating in November.
In a hypothetical general election matchup, the NBC/Marist poll showed Cunningham leading Tillis by 5 points, 48% to 43%.
Three Republicans are competing here for the chance to run against vulnerable Democratic Sen. Doug Jones, who is widely considered the most endangered senator up for reelection.
Tuesday's primary contest pits former Alabama senator and Trump Attorney General Jeff Sessions against Rep. Bradley Byrne and political newcomer Tommy Tuberville, a former Auburn University football coach. If no one gets a 50% majority on Tuesday, the top two finishers will face off later this month in a runoff.
With all three candidates polling close to one another, the race has the potential to be one of the most contentious primaries in the country, with all three candidates trying to run as close to Trump — who enjoys a 60% approval rating in Alabama — as they can.
It is also a highly risky venture for Sessions, who fell out of favor with Trump after he recused himself from Robert Mueller's Russia investigation. Sessions stepped down in 2018, and Trump has not endorsed his Senate bid.
Prior to joining the Trump administration, Sessions represented Alabama in Congress for three decades, first in the House and then the Senate. For Sessions, ending his political career with a loss in a primary runoff would surely tarnish his legacy.
If Tuberville comes out victorious, it could also pose fresh challenges for the Republican Senate majority. Tuberville is an Arkansas native who left Auburn in 2008 and has never run for public office before, so it's unclear how he would handle the intense scrutiny that comes with a major Senate campaign.
Powerful Republican Sen. John Cornyn is also up for reelection in November, and on Tuesday, Democratic voters there will elect his challenger.
While not nearly as vulnerable as Tillis or Jones, Cornyn is running in a state that is shifting from solidly conservative, to, increasingly, a toss-up.
And just like in North Carolina, here again the powerful DSCC has endorsed a moderate Democrat who consistently leads in the polls: MJ Hegar, a former Air Force helicopter pilot and entrepreneur.
The DSCC's decision to endorse Hegar in December sparked several of her challengers to accuse the Senate Democrats' fundraising arm of ignoring more qualified candidates of color.
The Texas primary is also expected to result in a run-off, potentially offering one of Hegar's challengers the opportunity to consolidate the opposition.
Both Cunningham in North Carolina and Hegar in Texas poll well among primary voters. But they also represent a centrist approach to 2020 that could backfire on Democrats, especially if Vermont Sen. Bernie Sanders is atop the party's ticket in November.
Sanders, a self-described democratic socialist, is running for the Democratic presidential nomination on an unabashedly populist and progressive platform, asking voters to reject political centrism and embrace his "movement."
And in both North Carolina and Texas, Democratic voters seem poised to do just that.
The NBC/Marist poll released Sunday showed Sanders leading his closest rival, Biden, by 15 points in Texas. In North Carolina, the race was tighter, with Sanders edging Biden out by only 2 points. The poll was taken before Biden's strong showing on Saturday in South Carolina's primary.