Super Tuesday: Will Haley Drop Out? Plus More to Watch For.

Voters in 15 states, including two titans, California and Texas, will head to the polls on March 5 for a Super Tuesday that is likely to set a White House rematch in November between President Biden and former President Donald J. Trump.

The contests will also determine the contours of races for the House and Senate that will shape the legislative branch next year.

Here is what else to watch as the results roll in.

Will Nikki Haley end her campaign, or keep going?

Ms. Haley, the former governor of South Carolina and Mr. Trump’s first ambassador to the United Nations, won her first Republican primary on Sunday, in the District of Columbia, and could pick up a few more on Tuesday. The moderate Republican senators of Maine, Susan Collins, and Alaska, Lisa Murkowski, endorsed her in recent days, just in time for their states’ Super Tuesday contests.

Minnesota’s open primary on Tuesday will allow Democrats to vote for Ms. Haley if they choose. And polling in Virginia showed her inching closer to Mr. Trump.

But the biggest troves of delegates — California has 169 and Texas 161 — are almost certain to go to the former president, and Super Tuesday has loomed large for Ms. Haley’s donors, who need to see she has a chance. More than a third of all delegates will be allotted on Tuesday, not enough to make Mr. Trump the presumptive nominee but enough to make him the prohibitive favorite.

Ms. Haley will then face choices with huge ramifications: Does she drop out and endorse Mr. Trump, drop out and hold off on any endorsement, stay in the race until her money is gone, or consider a third-party run? (She has said she will not do this, but the centrist group No Labels continues to hold out hope she would join its ticket.)

How will Mr. Trump handle victory?

The nation has watched two responses to victory by the former president and front-runner for a third Republican nomination. After he won the New Hampshire primary in January, Mr. Trump mocked Ms. Haley’s dress and castigated her for trying to make the most of her 43 percent, second-place finish. After his victory in Ms. Haley’s home state of South Carolina last month, he didn’t mention her.

Mr. Trump has made no secret of his desire to begin the general election campaign against Mr. Biden, and of his frustration with Ms. Haley’s stubborn insurgency, which has included harsh words for her former boss’s intemperance, age, fealty to the Constitution and loyalty to veterans and active-duty service members.

Expectations are for a big night for Mr. Trump. If he lashes out at a vanquished fellow Republican, he risks pushing some of her voters further away from him — and potentially toward Mr. Biden.

Will the pro-Palestinian protests dog Mr. Biden?

If anything, Mr. Biden faces even larger problems reuniting the coalition of voters who delivered his victory in 2020, but unlike the G.O.P., Democratic disharmony on Tuesday night will not manifest itself in votes for an alternative candidate. It may show in votes for “uncommitted.”

Even as Mr. Biden won overwhelmingly in Michigan last week, 13.2 percent of Democratic primary voters cast their ballots for “uncommitted,” most of them protesting the president’s tilt toward Israel in its brutal conflict with Hamas in Gaza. That total showed the fragility of the Democratic coalition — especially with young progressives and Arab Americans — as Mr. Biden begins a difficult drive toward re-election.

The next test for Mr. Biden comes on Tuesday in Minnesota. The state has far fewer Arab American voters than Michigan, but Minneapolis has a powerful progressive base. Leaders of the protest effort are hoping for 10,000 “uncommitted” votes, a fraction of the 101,436 who cast such votes cast last Tuesday. And Mr. Biden’s seven-percentage-point victory in the state in 2020 was more comfortable than his three-point win in Michigan.

But trailing in the polls, Mr. Biden needs to bring his party together, and pro-Palestinian voices understand they have leverage to try to sway U.S. policy in the war. His headaches will continue in Washington State on March 12, where progressives are mounting the next “uncommitted” campaign.

California’s consequential down-ballot primaries.

The largest state in the nation will hold the most consequential down-ballot primaries on Tuesday, thanks to its unusual primary system, in which the top two finishers face off on Election Day, regardless of party.

The marquee race is for the Senate seat held until last year by Dianne Feinstein, who died at 90 in September. The contest attracted three Democratic heavy hitters, all from California’s House delegation: Adam Schiff, Katie Porter and Barbara Lee.

For much of the campaign, it looked like the top two finalists would be Democrats, Mr. Schiff and Ms. Porter. Then came the rise of a celebrity Republican, the former Los Angeles Dodgers great Steve Garvey. He didn’t do much campaigning, but Mr. Schiff, figuring that in a Democratic state like California, a Republican would be easier to beat in November, spent $10 million on ads that ostensibly attacked Mr. Garvey as “too conservative for California” but intentionally elevated his candidacy.

On Tuesday, Mr. Schiff will see whether his strategy will work or whether Ms. Porter can eke out second place.

That primary system is also coming into play in a House seat in the Central Valley that Democrats dearly want to take from the Republican incumbent, David Valadao. The newly drawn district would have favored Mr. Biden by 13 percentage points in 2020, but before they get a chance to try to win it, Democrats have to contend with each other.

The party’s chosen candidate, a former assemblyman named Rudy Salas, is facing a spirited Democratic opponent in Melissa Hurtado, whose State Senate seat mirrors the U.S. House district. Both want to be the Central Valley’s first Mexican American representative, but if Democratic turnout is low and divided, Mr. Valadao could end up facing his Republican challenger, Chris Mathys, in November. Democrats will have blown one of their few shots to contest a Republican-held seat that favors Mr. Biden.

Redistricting cuts both ways.

House primaries in North Carolina and Alabama will show how the redrawing of district lines will help and hurt both parties as they fight for control of a House that Republicans control by three seats.

In North Carolina, the Republican super majority in the state legislature gerrymandered the map so thoroughly that a state with a Democratic governor and near 50-50 partisan split is likely to see its seven-to-seven House delegation swing to 10 Republicans, out of 14 total seats.

Three incumbent Democrats, Jeff Jackson, Wiley Nickel and Kathy Manning, decided not even to stand for re-election.

In Alabama, a Supreme Court ruling that the state’s Republican-drawn maps unconstitutionally deprived Black voters of representation forced new lines that will pit two sitting House Republicans, Jerry Carl and Barry Moore, against each other. Meantime, at least 11 candidates will fight for the newly drawn district, which is nearly 49 percent Black and would have been won in 2020 by Mr. Biden by more than 12 points.

In Texas, an impeached attorney general seeks revenge.

When Ken Paxton, the Republican attorney general of Texas, was impeached by a State House firmly in control of his own party, it looked like the ultimate nonpartisan rebuke.

The Texas House ultimately approved 20 articles of impeachment, by a lopsided vote of 121 to 23, related to accusations from a former top deputy that he had abused his office for the benefit of himself and an Austin real estate investor and campaign donor who was said to have assisted Mr. Paxton with home renovations as well as with helping Mr. Paxton conduct an extramarital affair. (Mr. Paxton declared the allegations false.)

Then last September, after a nine-day trial, the Texas Senate acquitted him. On Tuesday, Mr. Paxton is seeking vengeance on the Republicans who accused him.

Republicans aligned with Mr. Paxton or the state’s conservative governor, Greg Abbott, are challenging other Republicans in more than two dozen races. For good measure, Mr. Paxton is trying to remake the state’s highest criminal court by unseating three Republican judges who serve on the Court of Criminal Appeals.

If the challengers succeed, the nation’s largest, richest conservative state is likely to shift even further right.

Voters in North Carolina will set up a fierce governor’s race.

North Carolina has a peculiar habit of choosing Republican presidential candidates, Republican legislatures — and Democratic governors.

In 2024, with the current Democratic governor, Roy Cooper, term-limited and unable to seek re-election, Republicans hope to break that streak, though primary voters are likely to nominate a candidate who could prolong it. Mark Robinson, the state’s conservative lieutenant governor with a history of offensive and polarizing comments, including disparaging members of the L.G.B.T.Q. community, appears poised to win the nomination for the top post, setting up a contest with the Democrats’ likely choice: a mild-mannered, popular state attorney general, Josh Stein.

The race will be closely watched. North Carolina narrowly went to Mr. Trump in 2020, as Mr. Cooper was winning re-election. Mr. Robinson could get a boost from the presidential campaign — or Mr. Biden could get a boost from the governor’s race.