At a fundraising campaign stop outside Boston, Biden said "If Trump wasn't running, I'm not sure I'd be running," adding "We cannot let him win."
Reporters later asked Biden whether he would be running if Trump were not:
"I expect so. Look he is running, and I have to run."
Biden's comments come as even staunch Democratic voters have expressed concern in recent polls about the president's age.
At 81 years old, Biden is the oldest president in U.S. history.
Trump responded to Biden's remarks at a Fox News town hall held the same day, saying "somebody gave him a talking point they thought would sound good."
Trump and Biden last faced off for the Oval Office in 2020, after Trump served one term as president from 2017 to 2021.
Biden's aides increasingly regard Trump's frontrunner status for the Republican presidential nomination as insurmountable, according to two of those Democrats who declined to be named.
Recent polling has shown Trump leading Biden in hypothetical matchups in key swing states and on the national level.