In a race where many voters are following politics on their smartphones, Mr. Biden’s pullback is an unusual and potentially worrisome sign about his appeal among the Democratic activists, young people and donors who are especially engaged on social media. Candidates rarely withdraw so much money from their online campaigns unless they are seeing weak results in online fund-raising, according to interviews with digital strategists.

As the candidates make their final pitches to donors before a quarterly fund-raising deadline at midnight Monday, Senator Elizabeth Warren of Massachusetts and other top rivals are outspending Mr. Biden online. He was only the sixth-biggest Democratic spender on Facebook ads in the seven days ending Thursday, even as he was at the center of a swirling national controversy involving President Trump, Ukraine and impeachment…

“It’s clear that there’s something in the numbers that’s directing them to go in that direction, and the likeliest explanation is that the rate of return just wasn’t there for Facebook and Google,” said Tim Lim, a veteran Democratic digital strategist unaligned in 2020. “Which is very, very unusual.”

Zac Moffatt, a Republican strategist who led digital operations for Mitt Romney’s presidential run, said abandoning a medium used heavily by so many “reeks of desperation.”