President Biden couldn't possibly have a worse start to the NATO event -- he just screwed the pooch while introducing Ukraine's President ... in a moment where his mental acuity is under the microscope.

POTUS was introducing President Zelensky in Washington, D.C. when he flubbed and said, "Ladies and gentlemen, President Putin." Folks in the room had already started applauding, and continued right through the screw-up, which gave Biden time to recover.

President Biden quickly said, "President Putin? He's gonna beat Putin! President Zelensky!"

At any other time, the gaffe might be easily ignored -- however, Biden's mental agility, especially while not using a teleprompter, is something the whole world has been focused on ever since the debate against Donald Trump.

On the heels of prominent Democrats like George Clooney calling for Biden to drop out of the presidential race ... confusing Zelensky for his Russian nemesis, while on the world stage, isn't the look Biden needed.