As incumbent President Joe Biden and his Republican challenger, Donald Trump, prepare for their first debate later this week, one big question hangs over the US presidential race: who will Trump pick as his vice-presidential running mate?

There has been no shortage of speculation on this question. Will Trump choose another loyalist, nationalist type who is unlikely to expand his voter base? Or will he choose someone from the non-populist wing of the GOP in an effort to expand his extremely narrow lead over Biden?

There’s one logical choice for Trump if he wants to go this route. Whether he likes it or not, his best shot at winning could be his chief rival and critic until recently: former Republican presidential candidate Nikki Haley.

From critic to supporter

For months during the Republican primary contest, Haley had been unsparingly critical of Trump. She said, among other things, that Trump had “gotten more unstable and unhinged” since leaving office in 2021 and was “not qualified” to be president.

She also said: "He’s taking out his anger on others. He’s getting meaner and more offensive by the day. He’s trying to bully me and anyone who supports me."

Haley had been Trump’s strongest opponent in the primaries, winning contests in Vermont and Washington, DC. Notably, even after her withdrawal from the race in March, she continued to receive significant amounts of votes in states where Trump’s win was assured. These votes were seen as a protest against Trump – and a possible problem for him in the November election.

But last month Haley changed her tune. In a speech as the new chair of the Hudson Institute, Haley announced she would vote for Trump.

She said she wanted a president who’s going to have the “backs of our allies and hold our enemies to account”, and someone who would also secure the US-Mexico border. She noted Biden had been a “catastrophe” on these issues.

Her announcement divided Republicans. Former national security adviser John Bolton, an anti-Trump Republican, questioned Haley’s political calculations and whether she was angling to be his vice president. Republican strategist Sarah Longwell called her a “pathetic coward”.

Other Republicans praised the move, including David Wilkins, who served as US ambassador to Canada in the most recent Bush administration. He said: “Republicans need to be united as best we can.”

A history of sceptical Republican candidates

Haley’s pointed calls for robust American leadership role in the world – including support for Ukraine, Israel and Taiwan, and more open trade with like-minded, classically liberal nations – has made her the de facto leader of the internationalist (Trump supporters would say “globalist”) wing of the GOP.

This is a precarious position for someone who clearly wanted to be president. The Republican Party, when successful, has generally nominated presidential candidates who were sceptical of international entanglements.

In 1980, for instance, then-candidate Ronald Reagan ran on an agenda that included abrogating the Panama Canal treaty and questioning the newly normalised relations with China.

In 2000, George W. Bush ran against “nation building” and called for a more modest US role in world affairs.

In 2016, Trump expressed pointed opposition to the Iraq war and scepticism of the NATO alliance.

Once in office, of course, Reagan and Bush both shifted to a more internationalist approach.

Trump had more of a mixed record. He withdrew the US from the Trans-Pacific Partnership and the Iran nuclear deal and remained critical of American alliances. However, he didn’t pull the US out of NATO, as some feared, and actually improved American diplomacy in the Middle East by promoting the Abraham Accords between Israel and Arab nations.

How Haley could help

Trump can boost his campaign now by adding Haley’s internationalist credentials and voting base to his presidential ticket.

The Republicans successfully used this model in 1980 with an “America first” candidate at the top of the ticket (Reagan) and an experienced diplomatic hand (George H.W. Bush) as the vice-presidential candidate. (Both Bush and Haley are former US ambassadors to the United Nations.)

This option would, of course, depend on Trump’s willingness to invite Haley onto the ticket. Last month, Trump was more conciliatory towards his once-bitter rival, saying: "Well, I think she’s going to be on our team because we have a lot of the same ideas, the same thoughts."

While most anti-Trump Republicans will come back to the party in November when votes really count, Haley’s place on the ticket would ensure this. Her position as a possible vice president would also appeal to independent voters and perhaps even some Democrats who are upset with Biden’s performance in office.

The 2024 election is also likely to be Trump’s last campaign at the national level. If he wins, he will be a lame duck, unable to run again. (US presidents can only serve for two terms.) If he loses, he’ll be a spent political force and (likely) too old in 2028 to be a viable candidate.

So, after the 2024 election is settled, the Republican Party will begin looking to the future. Haley’s best – and probably only – chance of winning the Republican presidential nomination in 2028 is with Trump’s implicit endorsement as his running mate this year.