Millions of Americans had their say on Super Tuesday, and the country ended the day largely where it had started - expecting Joe Biden and Donald Trump to be the candidates in the election.

But lurking beneath the voting figures were warning signs - and a few lessons - for both the Democratic president and his Republican rival and predecessor.

Trump train rolls on

With the lack of suspense around the Republican presidential nomination for November's election, US cable network CNN dubbed the night "Stupor Tuesday."

As expected, long-time frontrunner Trump wiped the floor with his sole remaining opponent Nikki Haley, winning 12 of the first 13 states called, including the key swing state of North Carolina.

The former president will have to wait until March 12 or 19 to mathematically guarantee that he is the Republican standard-bearer in November.

But his emphatic victories all but guaranteed his rematch against Biden, firing the starting gun on what will be on the longest election campaigns in history.

Haley plays spoiler in Vermont

Haley was careful to keep expectations at rock bottom, refusing to predict victories in any state.

In the end she picked up Vermont, denying Trump a clean sweep and offering her supporters what will likely be no more than a consolation prize.

The former ambassador faces mounting calls to drop out and unite behind Trump.

The clamor for her to admit defeat will intensify after performances that did little to bolster her claim that she can lead 30-40 percent of the party who would rather see someone other than Trump challenge Biden.

While she was competitive in Virginia, Massachusetts and Colorado, her electability argument was undermined by a vote share that dipped below 20 percent in Texas, Tennessee, Alabama and Oklahoma.

Trouble for Trump?

Although Trump has swept all before him, returns in states where traditional Republican voters are turned off by chaos and scandal have trained a spotlight on potential vulnerabilities in key swing states come November.

In CNN's North Carolina exit poll, 66 percent of Haley voters said Trump was not physically or mentally fit to be president, while 81 percent said they would not automatically support the eventual nominee.

Despite Haley's overall poor showing, Republican strategist Karl Rove warned that the divisive tycoon "ought to be concerned about unifying the Republican party."

"There's still some work to be done," Rove said on Fox News, criticizing Trump for using nicknames for his opponents and threatening to excommunicate Haley donors.

Biden sees danger in Gaza protests

Biden hasn't really been tested by close rivals in the Democratic primary.

But Super Tuesday demonstrated again that his biggest threat may be the simmering discontent among many of the party faithful that could see them staying at home in November.

He faces a protest movement with its roots among progressive activists furious over his handling of the Israel-Hamas conflict in Gaza.

Alarm bells first rang when more than 100,000 Michigan Democratic primary voters cast ballots for "uncommitted" in Michigan's primary in February.

On Super Tuesday that figure was as high as 87,775 (12.7%) in North Carolina and around 45,000 in Minnesota (19%) and Massachusetts (nine percent), raising concerns that many younger left-leaning voters could desert him.

Reality check: there wasn't a huge difference between voters choosing uncommitted instead of Biden in 2024 and Democrats doing the same when Obama was seeking reelection in 2012.

And there was American Samoa

It wasn't just Trump that missed out on a clean sweep.

Biden has lost the Democratic caucuses in American Samoa - a South Pacific island territory of 45,000 people - to a White House hopeful who had actually visited as part of his campaign.

The US media hurried out explainer articles headlined "Who is Jason Palmer?" 

American Samoa has a history of contrarianism, having been the only state or territory to vote for Biden rival Michael Bloomberg in 2020.