Former President Barack Obama took aim at one of Donald Trump’s most well-known sore spots on Tuesday with a dig about crowd size.

“I heard there was another rally here on Friday night,” Trump said at an event for Vice President Kamala Harris at Detroit’s Huntington Place, where Trump spoke last week. “But it was a little smaller than this one.”

The crowd erupted:

Few things get under Trump’s skin quite like digs about crowd size.

Obama famously mocked Trump over it during his speech at the Democratic National Convention in August, complete with a memorable ― and suggestive ― hand gesture.

Trump the next day griped that Obama was “getting personal.”

Vice President Kamala Harris also took at shot at Trump’s crowds during their debate last month, pointing out that his audience is increasingly bored and heading for the exits while he’s still speaking.

The quip not only sidetracked Trump during the debate itself, but he’s also been complaining about it ever since ― even as he continues to have problems filling his venues.

At the Detroit rally, Obama pointed to Trump’s increasingly erratic behavior.

“If your grandpa was acting like this, you’d be worried,” Obama said. “This is somebody who wants unchecked power. We do not need to see what an older, loonier Donald Trump looks like with no guardrails. America’s ready to turn the page.”

Obama was introduced at the event by rapper Eminem ― and the former president responded by rapping a few lines of “Lose Yourself”: