Defiant Biden Throws Down Gauntlet To Democrats

US President Joe Biden insisted Monday he would not quit the election race, as the White House denied he had Parkinson's disease following a disastrous debate performance.

The 81-year-old dared Democratic critics to either challenge him at next month's party convention in Chicago or back him against Republican rival Donald Trump in November's vote.

The president lashed out in both a letter to Congress and a rare call to a television program, at the start of a critical week including a NATO summit in Washington where he will face fresh scrutiny.

"I am firmly committed to staying in the race," Biden wrote in the letter.

"It is time to come together, move forward as a unified party and defeat Donald Trump," he said. "It's time for it to end."

The embattled president followed up by phoning into MSNBC's "Morning Joe" television program to say he was "getting so frustrated by the elites" in the party.

"Any of these guys that don't think I should run -- run against me. Announce for president, challenge me at the convention," he added.

But even as he doubled down, the pressure mounted on the oldest president in US history.

Congressman Adam Smith, the senior Democrat on the US House Armed Services Committee, said Biden "should step aside."

"I think it's become clear he's not the best person to carry the Democratic message," he told CNN.

Biden's blitz was a clear attempt to lay to rest the spiralling concerns over his health following the June 27 debate against Republican Trump, whom he trails in the polls.

Biden repeatedly lost his train of thought, stared blankly and spoke with a raspy voice. Biden has blamed jetlag and a cold.

The White House has also felt the pressure, with tense exchanges at a press briefing on Monday.

Press Secretary Karine Jean-Pierre called for "respect" while journalists challenged her refusal to confirm reports that a Parkinson's specialist visited the White House eight times.

The visits by Kevin Cannard, a neurologist from the Walter Reed National Military Medical Center where Biden receives his medicals, were recorded in publicly available visitor logs.

Jean-Pierre denied that Biden was being treated for the condition, which can cause a stiff gait and affectless facial expressions.

"Has the President been treated for Parkinson's? No. Is he being treated for Parkinson's? No, he's not. Is he taking medication for Parkinson's? No," Jean-Pierre said.

The White House also denied reports that NATO allies attending this week's 75th anniversary summit in Washington had shown concerns about Biden.

"We're not picking up any signs of that from our allies at all," National Security Council spokesman John Kirby told reporters.

But NATO leaders have been seeking reassurance in any case amid polls forecasting a November victory for Trump, who has long criticized the defense alliance, voiced admiration for Russian strongman Vladimir Putin, and insisted he could bring about a quick end to the war in Ukraine.

The summit begins on Tuesday when Democrats, returning to Capitol Hill from a brief recess, hold a caucus meeting where Biden's fate will be discussed.

In recent days, five Democratic lawmakers have publicly called for him to drop out of the race -- with Smith becoming the sixth.

On Sunday US media said four senior congressmen -- including Smith -- said on a call with party lawmakers that it was time for Biden to bow out.

First Lady Jill Biden has fiercely defended her husband and did so again on Monday at the start of a trip to Georgia, Florida and North Carolina.

"For all the talk out there about this race, Joe has made it clear that he's all in," she at a veterans' event in Wilmington, North Carolina.

However the Democrat lags Trump in most polls despite his rival having recently been convicted of a felony in a porn star hush money case.

With election day just four months away and the party convention in mid-August, the clock is ticking on any move to replace Biden as the nominee.

Vice President Kamala Harris has been touted as a possible replacement, but she too has backed her boss.