The fastest-growing segment of the U.S. electorate is seniors. They supported President Trump in 2016 but aren’t squarely in his camp as the 2020 campaign picks up.

In 2016, despite polling showing an advantage for Democrat Hillary Clinton, voters over 65 backed Mr. Trump in the presidential election by a 52%-45% margin, according to exit polls. With still more than a year before next year’s election, 41% of seniors in a Wall Street Journal/NBC News poll in June said they would prefer to see Mr. Trump re-elected, while 48% favored a generic Democrat to win in 2020. That comes as 46% of seniors said they approve of Mr. Trump’s performance in office, slightly higher than his 44% overall approval rating.

Among 18-to-34 year-olds, Mr. Trump’s approval rating is just 33%, the Wall Street Journal/NBC poll shows, and Democratic candidates have paid great attention to young voters this cycle, announcing support for plans like student-debt forgiveness. But the fastest-growing generation of voters isn’t millennials, it is seniors. Voters over age 65 are projected to make up nearly a quarter of the electorate in 2020, the highest such share since 1970, according to the Pew Research Center.

“Given the antipathy that younger voters have toward [Trump], he really needs support among seniors to reach at least the support he received in 2016,” said Jeff Horwitt, a Democratic pollster with Hart Research Associates who helps conduct Wall Street Journal/NBC News polls.

Seniors are the likeliest to actually cast ballots, with two-thirds of them voting in the 2018 midterms compared with 53% of the overall voting-age population. While the electorate in presidential years skews younger than in midterm ones, no Democratic presidential candidate has won seniors since Al Gore in 2000, and for the past five presidential-election cycles, every Republican nominee has won a larger share of seniors than his predecessor.