Millions of Americans in 15 states and American Samoa are voting Tuesday, the biggest day of balloting in Democratic and Republican presidential nominating elections.

President Joe Biden and former President Donald Trump are expected to win wide approval from their party faithful en route to a November election against each other.

Neither Biden nor Trump can clinch their respective presidential nominations in the Tuesday voting.

But with more than a third of the delegates to this summer’s national party conventions at stake, Biden and Trump can take a huge step toward winning a majority of convention delegates and could soon clinch their party nominations in primaries in the next few weeks.

Biden has only token opposition in the Democratic contests, while Trump has one remaining challenger — former United Nations Ambassador Nikki Haley.

Trump has defeated her in every state primary or caucus contest so far, although Haley won Sunday in Washington, D.C., where a tiny contingent of 2,000 Republicans turned out to vote in the overwhelmingly Democratic national capital and made her the first woman to ever win a Republican presidential nominating contest.

But Haley faces a daunting reality on Tuesday, and her campaign is not predicting she will win any of the 16 contests. But state-by-state polling shows she could pull off an upset or two against Trump, even as he faces an unprecedented four criminal indictments encompassing 91 charges and one trial starting in three weeks.

Several of the states voting Tuesday are awarding all their party convention delegates to the winner of individual primaries, rather than a proportionate share based on the vote count, which is likely to greatly benefit Trump.

Polls show he is far ahead of Haley in Texas and California, the populous states with the two biggest hauls of delegates on Tuesday.

The question for Haley, if she loses most or all of Tuesday’s primary elections, is whether she drops out of the nomination race against Trump or soldiers on as a protest candidate, at least until Trump formally clinches the party’s presidential nomination for the third straight election cycle.

Trump won the presidency in 2016 but lost his reelection bid to Biden in 2020. National polling shows him with a narrow edge over Biden eight months to the day ahead of the 2024 presidential election.

In recent days, Haley has promised to stay in the Republican nomination race through the Super Tuesday voting, but she has not booked more television advertising in states with upcoming primary elections, and some of her major campaign donors have stopped sending money as her electoral losses have mounted.

While Biden is virtually uncontested in the Democratic primaries, political analysts are watching whether any sizeable number of voters in the midwestern state of Minnesota might cast ballots as “uncommitted” to protest his unwavering support for Israel in its war in Gaza. He is continuing his push for a six-week pause in the fighting.

Last week, about 13% of those voting in the Democratic primary cast “uncommitted” ballots in Michigan, where the country’s largest bloc of Arab Americans live.

Biden will also be in the spotlight Thursday night when he gives his annual State of the Union address before Congress and a national television audience.

He is expected to tout the U.S. economic advance during his three-plus years in office, although polls show many Americans do not think he has managed the economy very well and often complain about higher prices for groceries — the one consumer purchase no one can avoid.