As the incumbent, President Joe Biden was a shoo-in for the Democratic nomination once he announced he would seek re-election alongside Vice President Kamala Harris.
And among Republicans, former President Donald Trump has long been the frontrunner — and he rapidly and resoundingly trounced his challengers in the party primaries.
One question that has lingered, though, is who will be Trump’s pick for vice president.
While the vice presidency holds limited power in US politics, past elections have shown that the choice can be a boost — or a liability — to presidential campaigns.
This year’s Republican vice-presidential pick could be especially important, given the race between Trump and Biden is expected to be tight. Trump is expected to reveal his choice in the coming weeks, well ahead of the November 5 election.
Here, Al Jazeera looks at Trump’s possible running mates, their views and what they could bring to the 2024 Republican ticket.
Elise Stefanik, US congresswoman
When she was first elected to the House of Representatives in 2014, Stefanik was the youngest woman in history to join the chamber. She was 30 years old at the time.
She has since become a rising star in the Republican Party, representing New York’s 21st District.
Stefanik currently serves as the House Republican Conference Chair, the number-three leadership role for Republicans in the House. She is also one of Trump’s staunchest supporters in Congress.
Stefanik, who has backed Trump’s false claims that the 2020 election was stolen through widespread voter fraud, was among the 147 Republican lawmakers who voted against certifying Biden’s election victory.
She gained national attention earlier this year when she used a congressional hearing to hammer US university presidents over pro-Palestine protests on their campuses, accusing the administrators of allowing anti-Semitism to go unchecked.
But critics have noted that Stefanik herself has come under fire for discriminatory messaging. In 2021, for instance, her campaign released advertisements warning that immigration would “overthrow our current electorate and create a permanent liberal majority”.
That messaging, critics say, mirrored the so-called “great replacement theory”, a white supremacist conspiracy theory that falsely asserts that white people are being replaced as a result of a plot to increase non-white immigration. The theory was invoked in an attack on a Pittsburgh synagogue in 2018, one of the deadliest instances of anti-Jewish violence in recent US history.
In recent weeks, Stefanik has visited Israel to show support as the country wages a deadly war in the Gaza Strip. She has also slammed Trump’s conviction on felony charges in New York, saying the verdict was the result of a “rigged” justice system.
Tim Scott, US senator
The only Black Republican in the US Senate, the South Carolina politician entered the presidential race in May last year — but dropped out before the primaries even began, throwing his weight behind Trump.
The former president praised Scott as “a great politician” after receiving his endorsement.
Since then, Scott has been a prominent backer of Trump, defending the ex-president in scores of major US television news interviews and joining him on the campaign trail.
Earlier this month, US media outlets reported that Scott was launching an outreach effort worth more than $14m to get Black voters to support Republicans in key battleground states in November.
“The Republican Party, we offer you freedom. They [Democrats] offer you oppression,” he said in a recent social media video about what the GOP can offer communities of colour. “We offer mo’ money; they offer no money.”
On policy, Scott — who regularly invokes his Christian faith — has voiced support for law enforcement, pledged to protect religious liberty against what he said was an “assault” by the far left, and promised to reform the education system.
JD Vance, US senator
Vance jumped into US politics in 2022 after the success of his book, Hillbilly Elegy, which detailed his upbringing in the country’s Rust Belt.
A venture capitalist, Vance graduated from Yale Law School and served in the US military during the Iraq War before being elected to represent the state of Ohio in the Senate.
He has made a name for himself thanks to his fierce criticism of the Biden administration, particularly over issues like the economy and immigration.
His embrace of so-called “culture war” issues has also made him a conservative darling, particularly among Trump’s Make America Great Again (MAGA) base. He regularly denounces diversity, equity and inclusion (DEI) programmes that aim to address systemic racism and other forms of discrimination.
Vance has used his family history and early life to try to paint himself as a defender of white, working-class Americans.
Tom Cotton, US senator
Cotton — a US senator from the state of Arkansas — is outspoken on a range of domestic issues, but much of his focus has been on foreign policy.
He is a vocal defender of Israel and recently joined a group of US lawmakers who introduced legislation to sanction the International Criminal Court if it pursues arrest warrants for Israeli leaders accused of committing war crimes in Gaza.
Faced with concerns that Trump could curtail assistance to Ukraine if elected, Cotton defended the former president, saying he “supports Ukraine’s strength and survival” as it fends of a full-scale Russian invasion.
“I don’t think President Trump wants to prejudge what the situation will be come January, nor do I, in part because we have no idea how much worse Joe Biden can screw things up. We have to judge the circumstances as they exist, next year when [Trump] returns to office,” Cotton said on FOX News.
While Trump has been sceptical of continued US aid for Kyiv — highlighting divisions within the Republican Party itself — Cotton recently voted in favour of a massive bill that included $61bn for Ukraine.
Cotton is also a hawk on China and has led efforts in Congress to “decouple” the US and Chinese economies. Among his signature causes is an effort to sever the social media platform TikTok from its developer, the Chinese firm ByteDance, over national security concerns.
The Arkansas Republican drew criticism earlier this year when he pressed TikTok’s CEO Shou Chew on whether he had ties to the Chinese Communist Party during a congressional hearing.
In an exchange that went viral on social media, Cotton asked: “Have you ever been a member of the Chinese Community Party?” Chew replied, “Senator, I’m Singaporean. No.”
Doug Burgum, governor of North Dakota
A former software executive and multimillionaire, Burgum ended his long-shot bid for the Republican presidential nomination in December and endorsed Trump about a month later.
He had struggled to generate early momentum for his campaign, particularly in the lead-up to the Republican presidential debates last year.
His campaign initially stayed afloat by offering gift cards in exchange for campaign donations. That setup allowed him to meet the minimum number of donors to qualify for the party’s first two debates.
Trump praised Burgum’s business prowess after receiving his endorsement, telling a North Dakota radio station: “I’ve always had a lot of respect for Doug.”
As North Dakota’s governor, Burgum signed into law a near-total abortion ban, as well as legislation making it a crime to provide gender-affirming healthcare to most minors.
Burgum and other Republican legislators travelled to New York City last month to show support for Trump during his criminal hush-money trial. Echoing the ex-president himself, Burgum denounced the case as “election interference”.
Marco Rubio, US senator
The Florida Republican is one of the most experienced politicians on Trump’s shortlist for vice president — though the ex-president has a history of sparring with him.
The son of Cuban immigrants, Rubio has represented Florida in the US Senate since 2011.
He sought the Republican presidential nomination in 2016, facing Trump in the race. They frequently traded personal jabs during the primaries.
Rubio, for instance, called Trump a “con artist” and said he had turned the 2016 race into “one of the most vulgar spectacles in American political history”. Trump described Rubio as a “choker” and a “lightweight”. He also dismissively nicknamed the Florida senator “Little Marco”.
But Rubio has since embraced and defended the former president, calling Trump’s recent conviction in the New York hush-money case an “ugly, ugly blemish” on the US’s reputation.
The Florida senator has said he wants to “secure” the US-Mexico border and has backed a 15-week ban on abortion in the state. On foreign policy, Rubio comes from the hawkish wing of the Republican Party and has promoted hardline policies against Cuba, Iran and China.
Choosing Rubio as his vice president would present a minor complication for the Trump campaign: The 12th Amendment of the US Constitution states that presidential and vice-presidential nominees cannot be from the same home state.
Trump and Rubio are both Florida residents. But if Rubio is Trump’s pick, he could change his official residency before the election.