President Donald Trump's eldest son tried to mock Sen. Bernie Sanders' physical prowess -- but his attack backfired.

The Vermont senator and Democratic presidential candidate took a swing at a speed bag this week the Muhammad Ali Center in Louisville, Kentucky, but the 77-year-old got hit in the face by the bag -- and took a comically exaggerated step back before taking a couple more jabs.

Donald Trump Jr. used a video of the stunt in an attempt to mock Sanders as weak.

But other Twitter users pointed out the president and his son's physical blunders and general awkwardness.