Donald Trump’s grip on the Republican presidential nomination was all but certain to be confirmed on Tuesday, as 16 US states and one territory held primary votes. From Alabama to Alaska and from Arkansas to American Samoa, “Super Tuesday” represented Nikki Haley’s last chance to deny Trump his third nomination.

With both Biden and Trump on track to sweep their victories, both Biden and Haley took the moment to warn voters of the stakes.

Haley, the former South Carolina governor and UN ambassador, remained way off the pace, her only win in Washington DC and in need of a political miracle if she was not to be forced to end her campaign. But she doubled down on the reality of the election.

“We can do better than two 80-year-old candidates for president,” Haley, 52, told supporters on Monday in the suburbs of Houston, Texas.

Joe Biden is 81. Trump is 77. Polling shows majorities believe both are too old, but the problem is worse for Biden, even among Democrats and even as Trump has displayed as many public slips and gaffes.

Meanwhile, on Tuesday morning, Biden, speaking to DeDe in the Morning, a nationally syndicated radio show, said: “If we lose this election, you’re going to be back with Donald Trump. The way he talks about, the way he acted, the way he has dealt with the African American community, I think, has been shameful.”

Trump, for his part, told Fox News: “We have to beat Biden – he is the worst president in history.”

Early estimates, including from Minnesota and California, showed voter turnout lagging in several states, as voters were faced with a predetermined presidential rematch that did not appear to spark much enthusiasm, particularly among younger Democratic voters.

Frank Luntz, a Republican-aligned pollster, earlier told the Guardian that Super Tuesday “never mattered less”.

“I don’t know any political event that’s got more attention for being less relevant,” Luntz said. “The decision has been made. The choice is clear.”

Some of those who were voting planned to reject Trump and Biden and vote uncommitted, as part of a spreading protest over Biden’s handling of the Israel-Gaza war. The uncommitted campaign, which secured 100,000 votes in Michigan, has spread to multiple states, including Massachusetts and Colorado, and is expected to pick up the most substantial support in Minnesota, which also has a large Muslim community. The Democratic Socialists of America, a force with young progressives, has also endorsed the push for voters to choose “uncommitted”.

In Boston, 35-year-old Marwa Osman, a former Biden supporter, said she felt “betrayed” by the president’s Israel policy, and that she had voted “no preference” in the primary.

If the administration does not change its policy by November, Osman said, she may simply not vote, or vote for an independent party, she told the Associated Press.

“I just cannot have my vote in good conscience go to something that is aiding to kill innocent civilians,” she said.

Democratic primary voters’ anger and dissatisfaction with Biden was expected to have ripple effects on other races and potentially boost Republican candidates, as Democrats even in liberal states such as California chose not to cast their votes.

In the gilded ballroom of Mar-a-Lago in Florida, where Trump is gathering supporters and journalists for an election watch party tonight, the mood is likely to be triumphant, as the candidate cements his frontrunner status despite his legal battles across multiple states.

Trump received a boost on Monday, when the US supreme court ruled unanimously that judges in one Super Tuesday state, Colorado, erred when they said he should be kept off the ballot for inciting the January 6 insurrection. Maine, also voting on Tuesday, also attempted to stop Trump from running. The third state to do so, Illinois, holds its primary later in March.

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On the Democratic side of the ballot, Biden was all but sure to defeat his also-ran challengers, the Minnesota congressman Dean Phillips and Marianne Williamson, a self-help author. Amid deepening apprehension about the president’s age but also his record on the Israel-Gaza war, aides were chiefly concerned with turnout and protest vote totals.

Polling shows clear majorities of voters in both parties dissatisfied with the prospect of a Biden-Trump rematch, in which Trump will continue to deal with 91 criminal charges (for election subversion, retention of classified information and hush-money payments) and multimillion-dollar civil defeats.

“You know who the two nominees are and 70% of Americans would rather it not be so,” Luntz said.

Among voters, two self-identified Democrats in Arlington, Virginia, said they cast ballots for Haley, able to do so because the state holds open primaries, meaning voters do not have to participate in the primary of the party with which they are registered.

Although both voters said they planned to support Biden in the general election, they chose to vote in the Republican primary as a protest against Trump.

“There’s no greater imperative in the world than stopping Donald Trump,” said John Schuster, 66, after voting at Clarendon United Methodist church. “It’ll be the end of democracy and the world order if he becomes president.”

Schuster said he did not agree with Haley on most matters.

“It’s a vote against Trump,” Schuster said. “Nikki Haley is very conservative. I disagree with her on everything, except for on the issue of democracy and Russia. Anything to irritate [Trump] and slow him down is what I’m doing.”

There were other contests to watch.

In California, in an open primary, voters were deciding which two candidates for US Senate will advance to the November election. Adam Schiff, a Democratic former chair of the House intelligence committee, and Steve Garvey, a Republican former baseball star, led a crowded field.

In Texas, Democrats were choosing a candidate to challenge the Trump-supporting senator Ted Cruz. Colin Allred, a congressman and former NFL player, was the favourite.

In North Carolina, the Democratic attorney general, Josh Stein, and Republican lieutenant governor, Mark Robinson, were expected to advance to an election for governor. That race will decide who succeeds Roy Cooper, a term-limited Democrat, in a state of increasing importance for presidential elections and control of Congress.

Robinson, a rare Black Republican in elected office, has attracted widespread criticism for harsh rightwing rhetoric. At a rally last week, Trump called him “Martin Luther King on steroids”.

Rachel Leingang, Gabrielle Canon and David Smith contributed reporting