US - Joe Biden urged Democratic lawmakers Monday to either back his reelection campaign or challenge him at the party convention next month as the US president's reelection bid entered a critical week.
The 81-year-old doubled down on his defiance of calls to step aside, after a disastrous debate against Republican rival Donald Trump last month turbocharged fears that he is too old to serve a second term.
Biden will be under extra scrutiny this week as he hosts a summit of NATO leaders in Washington, with many allies seeking reassurance amid polls forecasting that the isolationist Trump will win in November.
Biden called in to MSNBC's "Morning Joe" television program to say that he was "confident" the "average voter out there still wanted Joe Biden."
Sounding angry in the telephone interview, Biden said 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.
In a blitz of action to start the week, Biden also penned a lengthy letter to Democratic Party lawmakers that saying "I decline" to stand down.
"I am firmly committed to staying in the race," Biden wrote.
"The question of how to move forward has been well-aired for over a week now. And it's time for it to end."
- NATO summit -
Biden must overcome concerns sparked by the debate, during which repeatedly lost his train of thought, mangled his syntax and spoke with a raspy voice.
He has blamed jetlag and a cold.
But on Sunday four senior congressmen said on a call with party lawmakers that it was time for Biden to bow out, according to US media.
Democrats this week return from a brief recess to Capitol Hill under pressure to either fall in line behind the president or urge him to step aside.
Tuesday, when the NATO summit begins, could prove a turning point: party lawmakers are expected to hold their regular caucus meeting, which could coalesce any bid to force him out.
Biden's performance at the summit marking the 75th anniversary of the NATO alliance will also be closely watched. He is due to attend several meetings and give his first press conference since the debate debacle.
Many European countries are fearful of a return of Trump, as the 78-year-old 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.
- Busy schedule -
After a barnstorming day of campaign events in swing state Pennsylvania on Sunday, Biden has no public events scheduled for Monday, which he is expected to spend preparing for the summit.
First Lady Jill Biden, a fierce defender of the president, is scheduled to campaign for him instead in Georgia, Florida and North Carolina.
On Friday the president picks up the campaign trail once more, heading for the battleground state of Michigan, before going to his beachside home in Rehoboth, Delaware.
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.
Biden and his team appear determined to dig in for the siege, with the campaign unveiling an intense schedule for later in July, including an avalanche of TV spots and trips to key states.
By Danny Kemp