President Joe Biden faced more calls from fellow Democrats to abandon his re-election bid on Friday, following a news conference in which he delivered nuanced responses but occasionally stumbled over his words.

It was unclear whether Biden's performance would convince doubters in his party that he is their best bet to defeat Republican Donald Trump in the Nov. 5 election and serve another four-year term in the White House.

At least 17 congressional Democrats so far have called for him to drop out and allow the party to pick another standard-bearer, including some who announced their positions after the news conference on Thursday night.

Democrats are worried that Biden's low public approval ratings and growing concerns that he is too old for the job could cause them to lose seats in the House and Senate, leaving them with no grip on power in Washington should Trump win the White House.

But Biden made clear that he did not plan to step aside.

"If I show up at the convention and everybody says they want someone else, that's the democratic process," Biden said, before shifting to the stage whisper he often uses for emphasis to add, "It's not gonna happen."

Biden perhaps did not reassure those who were spooked by his poor presidential debate performance against Trump on June 27.

At one point, he referred to his vice president, Kamala Harris, as "Vice President Trump". That came just hours after he introduced Ukrainian President Volodymyr Zelenskiy as "President Putin" at the NATO summit, drawing gasps from those in the room.

Biden occasionally garbled his responses at the news conference, yet he also delivered detailed assessments of global issues, including Ukraine's war with Russia and the Israel-Gaza conflict, that served as a reminder of his decades of experience on the world stage.

Some Democrats were not reassured.

"We must put forward the strongest candidate possible to confront the threat posed by Trump's promised MAGA authoritarianism. I no longer believe that is Joe Biden," said Representative Jim Himes of Connecticut, who called on the president to end his campaign after the news conference.

But one influential party figure, Representative James Clyburn of South Carolina, reiterated his support on Friday morning.

"I am all in. I'm riding with Biden no matter which direction he goes," he said on NBC's "Today" program.

A senior campaign official who spoke on condition of anonymity called the performance the "worst of all worlds. Not good. But not bad enough to make him change his mind ... It'll give some enough cover to back him publicly, only to say he's not up for it privately."

Fundraiser Dmitri Melhorn said other donors told him they saw a strong performance from the president. "This is the person who can beat Trump. The mistakes are baked in and the upside is strong," he told Reuters.

Biden will hold a rally on Friday in Detroit, where his campaign says he will focus on the "dangers" of Trump's agenda.

The Michigan city is also headquarters of the United Auto Workers labor union, whose leaders endorsed Biden but now are assessing their options, three sources told Reuters.

With most U.S. voters firmly divided into ideological camps, opinion polls show the race remains close.

An NBC/PBS poll released on Friday found Biden leading Trump 50% to 48%, a slight increase from his position before the debate. Biden fared slightly worse than Trump when third-party candidates were included in the questioning.

A Reuters/Ipsos poll released last week found Biden and Trump tied at 40% each. But some nonpartisan analysts have warned that Biden is losing ground in the handful of competitive states that will determine the outcome of the election.