Super Tuesday has over 40 years of history that are deeply entwined with a bunch of issues in the presidential nomination process. The term "Super Tuesday" was first used in 1980 but was promoted by Democrats in 1988 as a deliberate effort to give an advantage to more moderate candidates. This was a direct reaction to the idea that the party's 1984 nominee, Walter Mondale, had lost to Ronald Reagan because he was too liberal. This strategy didn't really work out, though, and Democrats ended up nominating another northern liberal, Michael Dukakis.

In other words, this early attempt at a Super Tuesday brought some of the disadvantages of a regional primary, reducing the amount of attention on individual states and pressuring states to move their primaries earlier without really strengthening the influence of the region. Another drawback was that, by trying to enhance Southern influence by holding contests early, Super Tuesday also contributed to the "front-loading" problem", or the issue of having too many delegates selected at the beginning of the primary calendar.

Over time, Super Tuesday has taken on other significance as an important turning point in the race. As party politics scholar Caitlin Jewitt noted, in 2016, everyone thought that Sen. Ted Cruz needed to do well on Super Tuesday, which was heavily concentrated in Southern states where he needed to rack up wins — and when that didn't happen, it was a significant blow to his candidacy. For Biden in 2020, Super Tuesday marked the point when the party consolidated behind him. And in 2008, Super Tuesday made it clear that the Democratic nomination was a real race between Hillary Clinton and Barack Obama, as neither one scored a decisive victory.

It's hard to know exactly how this will play out in 2024. Republicans don't have the same ideological history with the big primary day as Democrats do. But the potential for a turning point in the race is there: Because so many delegates are at stake today, unless Haley wins a significant share of them, it may not make sense for her to stay in the race after tonight.

—Julia Azari, 538 contributor

Since about the mid-1990s, Democrats have been electing more women to Congress than Republicans, and the difference grows bigger each cycle. This has a lot to do with the supply of candidates — more women identify as Democrats, and the women in the pool of traditionally "qualified" candidates (college-educated, in white-collar professions) are likely to lean Democratic. There's also the issue of demand — Democrats are more than twice as likely (75 percent to 29 percent) than Republicans to agree that there are too few women in politics.

These supply and demand issues may be mitigated if the Republican Party's organizational arm and donor class actively recruited, endorsed and financially backed women in primaries for competitive, or safe, red seats in November. This is the playbook Democratic PAC EMILY's List has been working from for years to elect more Democratic women to Congress and governorships. According to academic researchers, Democratic groups designated to elect more women are more likely to be prioritized by their donors than their Republican counterparts, which helps explain their success.

Although the GOP doesn't have a heavyweight equivalent to EMILY's List (which spends millions each cycle), there are groups committed to electing more Republican women to Congress. A couple of prominent new groups (Winning for Women and Elevate PAC) cropped up after the 2018 election, which elected 42 new women to Congress, but only four that were Republican. As a result, in both the 2020 and 2022 cycles, more Republican women ran in primaries than ever before, according to the Center for American Women and Politics.

But more women running doesn't always translate into more women winning. As we wrote ahead of the 2022 midterm elections, more Democratic than Republican women were nominated to run that year in House and Senate races where their party was either competitive or favored to win. While the GOP may have run and endorsed more women in primaries, it didn't emulate Democrats' strategy of actively recruiting women to run in races where they could win in November.

Today, there are just a few non-incumbent Republican women competitive for nominations in races they'd have any shot of winning in November. In Alabama's 2nd District, an incumbent-less primary due to redistricting, four of the eight Republicans running for the nomination are women. Of those women, attorney Caroleene Dobson has been endorsed by the women-focused VIEW PAC. Neither Trump nor the party committee has endorsed any candidate in that primary, but Dobson faces tough odds winning the crowded primary, and even tougher odds in a general expected to heavily favor Democrats.

11 Republicans are competing in Texas's 26th District, another race with no incumbent. Of two women in the race, Luisa del Rosal, a small business owner and former congressional chief of staff, has been endorsed by VIEW PAC. But she will have to defeat Trump endorsee Brandon Gill (who's also endorsed by the Club for Growth). The Republican woman running today with the most likely path to victory is former Rep. Mayra Flores: she's running to reclaim Texas's 34th District, the seat she won in a 2022 special election but lost in the general election later that year. She is endorsed by both Trump and VIEW PAC, which bodes well for her to face Democratic incumbent Vicente Gonzalez in a competitive general.

—Meredith Conroy, 538 contributor

Court-ordered redistricting seismically impacted the congressional map in Alabama, precipitating high-stakes primaries in the new 1st and 2nd Districts.

Under the new lines, the Democratic-leaning 2nd District runs from Mobile across much of Alabama's Black Belt. Overall, 11 Democratic hopefuls are on the ballot, but State Rep. Napoleon Bracy, state House Minority Leader Anthony Daniels and former Justice Department official Shomari Figures may be best-positioned to advance to a likely April 16 runoff. Through mid-February, Figures had raised about $299,000, second only to Daniels's $323,000. But Protect Progress, a pro-cryptocurrency super PAC backing Figures, has shelled out $1.7 million to support him, according to data from OpenSecrets. By comparison, Bracy had only raised around $106,000. Still, Bracy may have geography working for him: His home base of Mobile County constitutes 36 percent of the district's population, more than any other county.

The new map also placed two second-term GOP incumbents into a head-to-head primary for the new, dark red 1st District: Rep. Jerry Carl, who currently represents about 59 percent of the new seat's population, and Rep. Barry Moore, who represents the other 41 percent. In such a deeply Republican seat, both contenders have tried to position themselves as the truest pro-Trump conservative. Carl has had the upper hand monetarily. Across his campaign and victory committees, Carl has brought in more than $2 million, compared with Moore's $688,000. And Carl is also getting somewhat more outside support, with groups spending $1.5 million to oppose Moore and just $1.1 million to support Moore or oppose Carl. A late February survey from Auburn University at Montgomery found Carl ahead 43 percent to 35 percent, perhaps in line with Carl's slight geographic and monetary advantages.

—Geoffrey Skelley, 538

Yeah, Nathaniel, polling suggests that those endorsements could make a real difference. According to a February poll from the University of Houston Hobby School of Public Affairs, 70 percent of Republican primary voters said that Trump's endorsement would make them more likely to support a state-legislative candidate, and 64 percent said the same of Abbott's endorsement. In addition, 60 percent said that they would be less likely to support a candidate who voted against Abbott's school voucher legislation. And in a YouGov/University of Texas poll from February, a plurality of Republicans (37 percent) said that the Texas House was not justified in impeaching Paxton, compared with 28 percent who said it was justified and 35 percent who weren't sure.

—Mary Radcliffe, 538