President Joe Biden, facing a political crisis as Democrats question the viability of his campaign and mental fitness, will be put to the test on Thursday when he holds his first solo news conference of the year -- and the first since his debate two weeks ago.

The high-stakes moment on Thursday evening is an opportunity for Biden to change the narrative after his poor debate performance triggered a drumbeat of concerns in his own party that he might be too weakened to win against Donald Trump this November.

But any stumbles in the unscripted setting could add fuel to the fire, despite Biden's repeated attempts to rebuff his critics and his insistence that he is staying in the race.

Many Democrats have said they need to see Biden clearly answer questions without faltering or losing his train of thought -- what so alarmed them about his debate showing with Trump.

The news conference, coming just as network newscasts first come on the air, will come after Biden concludes hosting a NATO summit in Washington.

Biden kicked that off with a strong speech on the strength of the alliance, which is marking its 75th anniversary, and an announcement of new air defense capabilities for Ukraine in its fight against Russia.

But overshadowing the international gathering was Biden's domestic political fate amid debate over his ability to lead the U.S. for another four years and the possibility of a second Trump presidency.

Those questions have been top of mind for congressional Democrats this week as they returned to Washington after the holiday weekend and have been huddling behind closed doors to discuss the path forward.

Biden tried to preemptively block criticism in a defiant letter to Democrats on Monday, in which he said it was time to "come together" and that he is "firmly committed" to staying in this race to the end.

But there are now 13 House Democrats and one Democratic senator who have called on Biden to resign. Privately, many have expressed concern about the possibility of not only losing the presidency but also the House and Senate if Biden remains atop the ticket.

House Minority Leader Hakeem Jeffries, D-N.Y., told his colleagues he plans to relay their concerns about Biden's campaign to Biden himself, multiple people familiar with his comments told ABC News.

Jeffries and Senate Majority Leader Chuck Schumer have publicly backed Biden. Schumer simply told reporters, "I'm with Joe" when he was peppered with questions on Biden's candidacy.

But seeming doubt from one of the most prominent Democrats in Washington, former House Speaker Nancy Pelosi, prompted a fresh wave of outcry and concern.

When asked during an appearance on MSNBC if Biden had her support, Pelosi said it was "up to the president to decide if he is going to run" and that she wants him to "do whatever he decides to do."

In the whirlwind days following the debate, Democrats urged Biden to do more public appearances and off-the-cuff exchanges.

Biden sat down for his first post-debate television interview with ABC News anchor George Stephanopoulos in an effort to course correct, during which he largely dismissed broader concerns about his fitness and said he would only drop out if "Lord Almighty" told him to.

Thursday's news conference will be Biden's first solo one since November, though he's held joint news conferences with various world leaders four times since then. The White House, while not stating how many questions Biden will take, said it will be more than what is allotted during a joint presser.

"He’s looking forward to it," White House press secretary Karine Jean-Pierre told reporters. "And he will be taking your questions. So, that’ll be a good thing."