International Desk

Joe Biden and Donald Trump cruised to comfortable victories on Super Tuesday, setting up an almost certain rerun of the 2020 election, but their primary successes cloak problems that could have a significant impact on the White House race.

Worries over Biden's age and anger over his handling of the Israel-Hamas war have muted celebrations over his big wins while Trump faces doubts over his ability to unify a party fractured like no time in modern history by his attacks on the Republican establishment.

Third-party and independent candidates have meanwhile emerged as a potentially critical factor, with voters -- turned off by the two historically unpopular frontrunners -- apparently more open than ever to wildcards who could tilt the election.

If the primaries held so far have demonstrated anything, it's that the Republican Party is not united behind its now-presumptive nominee.

Substantial anti-Trump resistance has cropped up among moderate suburban voters in almost every state to hold a presidential nominating contest.

Crucially, around two-thirds to three-quarters of his rival Nikki Haley's voters in South Carolina, Iowa and New Hampshire made clear in exit polls that Trump could not count on their vote.

"Haley's quixotic race for the GOP nomination exposed Trump's flawed and weakened standing within the Republican Party, but more broadly with the American electorate," Alyssa Farah Griffin, a communications director in the Trump White House, wrote Wednesday in an op-ed for The Daily Beast.

- 'Alarm bells' -

Trump's criminal exposure -- 91 felony charges across four jurisdictions -- has further complicated his path, with at least one in three Republican primary voters ruling out supporting a convicted felon.

Trump said Monday on social media that he wants to unite Republicans, but at the same time gloated over how he had "trounced" Haley and accused her of having received support from "radical left" Democrats.

Biden's troops, meanwhile, are lukewarm about their own standard-bearer for different reasons.

A recent New York Times/Siena College poll revealed that the majority of the 81-year-old president's 2020 voters believe he is "too old to be effective."

"If I'm in team Biden's camp right now, those alarm bells, they are sounding, and they are sounding loudly," Republican strategist Colin Reed told Sky News.

While there's not much he can do about his age, another vexing issue is a growing protest movement among Democrats over the war in Gaza.

In Michigan -- a state Biden won in 2020 but Trump took home in 2016 -- more than 100,000 Democratic voters cast ballots for "uncommitted" in the February primary. Figures of 45,000-88,000 in several Super Tuesday states piled on the agony.

"These are unbelievable numbers," Hassan Abdel Salam, a founder of the Abandon Biden movement told AFP, adding that he was seeing support from Jewish and African Americans, as well as Latinos and young voters.

Biden supporters however point to similar numbers in some of those states when Obama sought reelection in 2012.

- The Kennedy question -

But it is still a headache the Biden campaign could do without in an election that is expected to come down to a few hundred thousand votes across a handful of swing states.

Biden and Trump may be able to tamp down rebellions within their ranks by November, leaving what could turn out to be the biggest threat of all: third-party candidates.

Americans haven't elected an independent president since George Washington, but a public turned off by a Biden-Trump rematch appears more open than ever to the idea of a wildcard who could tilt the election.

Democrats are particularly sensitive to the threat, recalling how Green Party standard-bearer Jill Stein threw a spanner in the works for Hillary Clinton in 2016.

Some analysts believe that John F Kennedy's nephew -- Robert F Kennedy Jr. -- could shape 2024 into a genuine three-person race, pointing doubters to the multitude of pundits who gave Republican Trump no hope in 2016.

"RFK is going to be an issue for Trump and Biden because it allows people to go somewhere else," said political consultant Keith Nahigian, a veteran of numerous campaigns and an official in George HW Bush's White House.

"It is up to both campaigns to keep people in their lane."