US President Joe Biden held a much-anticipated news conference on Thursday evening that many in his own party were viewing as a crucial test as to whether he should remain the Democratic candidate in November.

While Biden did make one notable flub when he referred to Vice-President Kamala Harris as "Vice-President Trump", along with saying "anyway" and stopping several of his responses mid-sentence, the early perception was that his showing could slow the clamor for him to step aside. More Democrats, however, did call for him to step aside afterward.

The animosity between Biden and former president Donald Trump played out on social media when Trump wrote on his Truth Social: "Crooked Joe begins his 'Big Boy' Press Conference with I wouldn't have picked Vice President Trump to be vice president … Great job, Joe!"

Biden later replied on X: "By the way: Yes, I know the difference. One's a prosecutor, and the other's a felon."

Trump was also criticized frequently by Biden during his responses to reporters, saying the former president was busy golfing.

Biden misspoke earlier Thursday at the NATO summit when he referred to Ukrainian President Volodymyr Zelensky as "President Putin", although he quickly corrected himself.

A reporter asked if he felt the gaffe along with doubts about whether he should continue in the race could damage US standing in the world.

"Do you see any damage by me leading this conference?" Biden said. "Have you seen a more successful conference? I was talking about Putin and I said — at the very end — I said, 'Putin. I'm sorry, Zelensky.'"

Biden also said he would "pace myself" more as he campaigns.

The start time of the news conference, Biden's first by himself in eight months, was pushed back until after 7 pm. The president spent the first eight minutes of the roughly hourlong press conference talking about the NATO summit in Washington, although he read from a teleprompter. When it came time for questions from the media, Biden had a preselected list of 10 reporters.

The pressure was on the 81-year-old president to demonstrate he was capable of serving another term following his widely panned performance in a debate with Trump on June 27.

A follow-up interview that Biden had with ABC News' George Stephanopoulos on July 5 didn't do much to quell unease among Democratic politicians and partisan Hollywood celebrities. The president variously attributed it to a "bad night", a cold and jet lag, but many critics were unconvinced by those assertions.

So far, 17 of the 213 Democrats in the House of Representatives and one of the Senate's 51 Democrats have appealed publicly to the president to withdraw from the race.

US Representative Jim Himes of Connecticut, the top Democrat on the House intelligence committee, said after the news conference that he hopes Biden will step away from his campaign.

Himes said in a post on X that it has been "the honor of my career" to work with Biden, but that "the 2024 campaign will define the future of American democracy, and we must put forth the strongest candidate possible to confront the threat posed by Trump's promised MAGA authoritarianism".

Two other Democratic congressmen, Scott Peters of California and Eric Sorensen of Illinois, also called for Biden to abandon his bid late Thursday.

United Auto Workers union officials met on Thursday to discuss their concerns with Biden's candidacy, three sources familiar with the matter said, after endorsing Biden in January, Reuters reported. The 400,000-member union has a big presence in industrial states like Michigan that Biden will need to carry to win reelection.

Asked if he would consider bowing out before November if he's shown data that Harris could perform better against Trump, Biden responded, "No, unless they came back and said, 'There's no way you can win.'"

"No one's saying that," Biden said. "No poll says that."

He conceded that there were probably other candidates "who can beat Trump", but pointed to his campaign's strong fundraising and operations in battleground states. He noted that any other candidate would have "to start from scratch".

Biden was asked if he would be able to take a meeting with President Vladimir Putin and replied, "I'm ready to deal with him now and three years from now.

"I'm dealing with Xi right now and in direct contact with him," Biden said, referring to Chinese President Xi Jinping. "I have no good reason to talk to Putin right now… but there isn't any world leader I'm not prepared to deal with."

Foreign policy was deemed as Biden's strong suit by some supporters after the press conference.

"I think he convinced a lot of people he should stay in the race," said Representative Steve Cohen, a Tennessee Democrat, in a CNN interview. "I haven't heard anybody speak so much and so well and so knowledgeably about foreign policy since maybe I don't know … (former secretary of state) Dean Rusk."

"This is a very strong performance. … @POTUS is putting on a master class in how foreign policy and domestic policy intersect, explaining how crucial American global leadership is to our people here at home. Well done, Mr. President," Democratic strategist Joel Rubin posted on X.

Agencies contributed to this story.