A defiant President Biden on Friday declined to agree to an independent neurological assessment and suggested the only thing that might persuade him he could lose to former President Donald Trump is if the "Lord Almighty" came down and told him so. 

In Mr. Biden's first interview since the debate, ABC News anchor George Stephanopoulos gave the president multiple opportunities to answer whether he would agree to an independent cognitive or neurological examination, given widespread concern after his lackluster debate performance. Instead, the president said he has a cognitive test "every day," and his test is "running the world." 

"Look, I have a cognitive test every single day," Mr. Biden said, insisting the rigors of the job are sufficient to prove his mental fitness. "Every day, I have that test. Everything I do. You know, not only am I campaigning, but I'm running the world." 

The president reiterated that he believes he'll be fit for the job for the next four years. Stephanopoulos asked the president if he's being honest with himself about his capabilities. 

"Yes, I am, because George, the last thing I want to do is not be able to meet that," Mr. Biden said. 

Stephanopoulos asked the president whether he would step aside if he could ever be convinced he won't defeat Trump, since the president said he doesn't believe polls that show Trump would win. 

"It depends on — on if the Lord Almighty comes down and tells me that, I might do that," he laughed, adding later that "the Lord Almighty is not coming down."

Mr. Biden said that he has spoken to Democratic leaders in Congress "in detail," and he said all of them said he should stay in the race. He did not entertain the possibility that any would ask him to step aside from the nomination.

"They're not going to do that," he told Stephanopoulos. He said he had an hour-long conversation with House Minority Leader Hakeem Jeffries and had spoken "many hours" with Senate Majority Leader Chuck Schumer, and he also referenced his meeting this week with the Democratic governors.

Stephanopoulos pressed him further, asking if told by friends and supporters that staying in the race would mean losing the House and the Senate, what would he do?

"I'm not going to answer that question," Mr. Biden replied. "It's not going to happen."

He acknowledged that while there may be those who want him to leave the ticket, "I say the vast majority are not where those folks are."

Mr. Biden, asked how he'd respond if Trump were elected, and all the things he has warned about come to pass, said that as long as he had given it his all and had done the best job he knew how to do, "that's what this is about."

The president insisted his his poor debate performance was simply a "bad episode" in which he was "feeling terrible," and said he doesn't think he's watched the debate on screen himself. 

"It's a bad episode," he said. "No indication of any serious condition. I was exhausted, I didn't listen to my instincts in terms of preparing, and a bad night."

Stephanopoulos pointed out that the president had returned from rigorous travel in Europe a week and a half before the debate, and spent six days at Camp David resting and preparing. So why did he still perform so badly? 

"Because I was sick, I was feeling terrible," Mr. Biden said. "...I just had a really bad cold." 

Stephanopoulos asked Mr. Biden if he's gone back and watched his performance, a performance that sent the Democratic Party into crisis mode. 

"I don't think I did, no," he responded. 

The ABC News anchor pointed out that the president seemed to struggle from the moment the debate began. 

"Well, I just had a bad night," Mr. Biden said, repeating a line that many of his defenders and his press secretary have frequently used in the last week. 

Stephanopoulos asked the president when he realized he was having a bad night. The president replied that he recognized this when Trump was yelling or speaking loudly, and the microphone was turned off. The president said he let Trump distract him. 

"I just realized that I wasn't in control," Mr. Biden said. 

In the debate, which was viewed by over 50 million Americans, Mr. Biden struggled not only to rebut former President Trump's arguments, lies and misstatements, but also had trouble with answers about his own administration's policies, speaking softly, rambling and losing his train of thought. He moved stiffly and appeared to be frailer than his opponent.   

"Nobody's fault but mine," Mr. Biden said of his struggles. 

The president has been fighting to make the case that he's still up to the job, as Democrats publicly and privately voice their concerns about whether he should remain the Democratic presidential nominee. As concern within the party grew, he met with Democratic governors at the White House Wednesday, after the president did little personal outreach to top Democrats the weekend after his lackluster debate performance. 

Several Democratic House members have so far publicly called on him to drop out of the race — Reps. Lloyd Doggett, Raul Grijalva, Seth Moulton, Mike Quigley and Angie Craig. 

In an interview with CBS News Friday night, Quigley said that Mr. Biden's post-debate comments have not allayed his concerns. 

"Now is the time to be candid with ourselves. I appreciate loyalty. This isn't about loyalty, this is pragmatic politics to avoid a second Trump term."

Mr. Biden was defiant during a campaign rally in Madison, Wisconsin, Friday afternoon, speaking to elected Democrats and the donor class as much as to Wisconsin voters. 

"I'm the nominee of the Democratic Party," he said. "...You voted for me to be your nominee — no one else. You, the voters, did that. And despite that, some folks don't seem to care who you voted for. Well, guess what, they're trying to push me out of the race. Well, let me say this as clearly as I can: I'm staying in the race."

Asked after the speech whether he's considering dropping out or ruling out that possibility, Mr. Biden responded, "Completely ruling it out." 

Any decision to leave the race would have to be voluntary. Democratic Party leaders can't push Mr. Biden off the ticket because he has already won the delegates necessary to clinch the nomination.

Vice President Kamala Harris is standing by the president. 

"Look, Joe Biden is our nominee," she said in an exclusive interview with CBS News earlier this week. "We beat Trump once, and we're going to beat him again, period."

Kathryn Watson

Kathryn Watson is a politics reporter for CBS News Digital, based in Washington, D.C.