Millions Head to Polls on a Day That Will Shape the November Vote

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.