US President Joe Biden on Wednesday faced more doubts about his reelection campaign from lawmakers from his Democratic Party.

Late Wednesday, Senator Peter Welch called on Biden to withdraw from the election, making him the first Democratic senator to do so.

Biden is running for reelection in November, where he is likely to face off against Republican former-President Donald Trump.

The challenges come as 81-year-old Biden welcomes NATO leaders in Washington.

What did Democrats say about Biden's campaign?

In an opinion piece for the Washington Post, Welch pointed to Biden's performance during a debate with Trump in which the US president spoke in a brittle voice and appeared to lose his train of thought several times.

"We cannot unsee President Biden's disastrous debate performance," Welch said. "For the good of the country, I'm calling on President Biden to withdraw from the race".

Also on Wednesday, two more Democrat lawmakers from the House of Representatives, Pat Ryan and Earl Blumenauer, publicly asked Biden to withdraw his candidacy.

Meanwhile, former House Speaker Nancy Pelosi stopped short of calling for Biden to stay in the race, saying, "It's up to the president to decide" whether to run.

"I want him to do whatever he decides to do," Pelosi told the MSBNC broadcaster when asked whether she wants Biden to stay in the race.

"We're all encouraging him to make that decision because time is running short," Pelosi said.

Hollywood star George Clooney, a major fundraiser for the Democratic Party, also on Wednesday said Biden should drop out of the race in a New York Times opinion piece.  

Biden has vowed to continue on with the campaign and has dismissed suggestions he is suffering cognitive decline, describing his debate performance as a "bad episode."

On Thursday, Biden is expected to take part in a press conference with reporters, a moment for the public and the Democratic Party to gauge his mental acuity.

House panel summons Biden aides

The Republican-led Oversight Committee in the House of Representatives subpoenaed three senior White House aides for questions regarding Biden's health and mental fitness.

The panel subpoenaed Anthony Bernal, a top aide for Biden's wife, Jill; deputy chief of staff Annie Tomasini; and senior adviser Ashley Williams.

"Key White House staff must come before our committee so we can provide the transparency and accountability that Americans deserve," Republican lawmaker James Comer said, calling Biden "unfit" for office.

"According to one former Biden aide, these three employees — Annie Tomasini, Anthony Bernal, and Ashley Williams — have created 'a protective bubble around' President Biden," the committee claimed.

White House spokesperson Ian Sams dismissed the move as a "baseless political stunt" and accused Comer of using the subpoena to "get into headlines."

sdi/wd (AP, AFP, Reuters)