(Reuters) - Gina Gannon, a retiree in the battleground state of Georgia, voted for Republican Donald Trump in 2016 before ditching him for Democrat Joe Biden in 2020 - and is now looking to next week's debate to help her decide which one to back this year.

Gannon, 65, flipped to Biden, she said, because she felt Trump's presidency was too chaotic. But she is now leaning toward Trump again, unhappy about illegal immigration at the U.S.-Mexico border and inflation under Biden's administration.

Biden could sway her, however, if he presents a strong proposal to secure the border and shows a steady hand despite his age, she said.

Trump, 78, is "always a wild card," Gannon said. Yet for the 81-year-old Biden "there's certainly the age concern and how he will be able to handle himself."

About 20% of voters say they have not picked a candidate in this year's presidential race, are leaning toward third-party options or might not vote at all in the Nov. 5 election, according to the most recent Reuters/Ipsos poll.

Reuters interviewed 15 such voters ahead of the June 27 debate in Atlanta to learn what they hope to see when Biden and Trump square off and how the candidates - now essentially tied in national opinion polls with fewer than five months until Election Day - can earn their support.

The group of undecided voters includes seven men and eight women from a mix of Democrat-leaning, Republican-leaning and battleground states. They vary by age, party affiliation and race.

Of the 15 voters, nine were previous Biden voters who had partially or fully soured on him, with one now leaning toward Trump. Three of the 15 voters had soured on Trump but were not considering Biden as an alternative.

Mental fitness, especially Biden's, is a key issue for these undecided voters, who will be watching the first debate to see how well the two oldest candidates ever to run for U.S. president can think on their feet.

Biden's stewardship of the economy, and especially his handling of inflation, is also a hot topic.

Increases in consumer prices have slowed considerably from a peak in June 2022, but voters still regularly complain of sticker shock at the grocery store.

Pennsylvania resident Rich Liebig, 35, voted for Biden in 2016 and 2020 but is now undecided, chiefly because he feels Biden is too old. Liebig also is put off by Trump's legal problems and what he called the "hullabaloo" around the Republican former president.

Liebig, who was laid off from his job in marketing recently, said he will watch the debate for signs from Biden on the economy.

"What is his agenda, if he gets a second term, to address inflation?" Liebig said, adding that he also wants to see strength from the president. "Biden has got to show that he can take on Trump again."

FOCUS ON IMMIGRATION

Several voters who supported Biden in 2020 said he needs to address immigration during the debate. Biden took office in 2021 vowing to reverse many of Trump's restrictive border policies, but he has struggled with record numbers of migrants caught illegally crossing the U.S.-Mexico border on his watch.

Biden has shifted rightward on the issue and earlier this month instituted a broad asylum ban to reduce illegal crossings. Trump, who made a hardline stance on immigration a centerpiece of his 2017-21 administration, has vowed a wide-ranging crackdown if reelected.

The border "needs to be under control," said Ashley Altum, a mental health case manager in South Carolina who voted for Democrat Hillary Clinton in 2016 and Biden in 2020, and thinks both Biden and Trump are too old to be running.

Altum said she would never consider voting for Trump but might have backed a different Republican candidate. Now, she may abstain from voting at all in the presidential election.

Trump's felony conviction in May is a factor for voters like ShaRon Johnson Bynum, a former Biden supporter who is unhappy with the Democratic president but believes the conviction disqualifies Trump.

Bynum, a 59-year-old telecommunication program manager in North Carolina, voted for Clinton in 2016 and Biden in 2020 because she felt Trump was unfit for the role. A registered independent, she said she has voted for Republican presidential candidates in the past.

This year, as she decides whether to vote for Biden or a potential third party candidate, Bynum said she will watch the debate to see if Biden is "mentally and physically able to do the role."

"My biggest concern right now is these trials and the outcome of these trials for the Republican candidate," Bynum said, referring to Trump's three remaining criminal cases. "And the age and capacity of the Democratic candidate."

Tom Reich, a 39-year-old Republican in Maryland, did not vote for president in either 2016 or 2020 and is open this year to voting for independent candidate Robert F. Kennedy Jr., who has not qualified for the debate.

Reich said he will be looking to see if either of the main party candidates shows signs of mental decline as they square off: "Anything that either says that is way out in left field, doesn't make sense and is not the kind of thing I'd want to see from someone running the country," he said.

"Anything scary in either direction would sway me toward the other," Reich said. "I think that is more likely than either candidate swaying me toward them."