(The Hill) — President Biden on Thursday mistakenly referred to Ukrainian President Volodymyr Zelensky as President Putin — the president of Russia — when introducing him at a NATO event, a gaffe that came shortly before a high-stakes press conference where his every word will be closely scrutinized.

“Now, I want to hand it over the president of Ukraine, who has as much courage as he has determination. Ladies and gentleman, President Putin,” Biden said.

“He’s going to beat President Putin. President Zelensky,” Biden said, quickly correcting himself. “I’m so focused on beating Putin, we’ve got to worry about it.”

“I’m better,” Zelensky quipped.

“You are, a hell of a lot better,” Biden added.

Support for Ukraine has been a cornerstone of Biden’s foreign policy, and it has been central to discussions during this week’s NATO summit. The White House has announced additional military support for Ukraine in its war against invading Russian forces as part of the summit.

But Biden’s stumble will garner significant attention as he fends off calls from some Democrats to step aside as the party’s nominee in November following his disastrous debate performance last month. Democrats have expressed concerns Biden lacks the mental sharpness and stamina to vigorously campaign against former President Trump and make a successful case to the American public.

Democrats will be closely monitoring Biden’s press conference on Thursday evening to see if he shows he is up to the job of coherently and sharply answering questions from reporters. But some have suggested one good performance will not be enough to quell voters’ concerns.