Joe Biden came out swinging on Monday against critics of his calamitous June debate performance, telling Democrats in an open letter and Americans in a pugnacious live TV interview he is staying in the presidential race – rejecting growing calls to concede that at 81 he is too ineffective to beat Donald Trump and should drop out in favour of a younger candidate. The president lashed out at “elites in the party” in a live telephone interview with the MSNBC show Morning Joe, saying they were behind calls for him to quit. He added: “If any of these guys don’t think I should run, run against me. Go ahead. Announce for president – challenge me at the convention!” Touting what he said was a demanding campaign schedule around hosting a Nato summit this week, the president insisted: “The American public is not going to move away from me. “I’m here for two reasons, pal. One, to rebuild the economy for hard-working middle class people, to give everybody a shot. It’s a straight shot. Everybody gets a fair chance. Number two, people always talk about how I don’t have the wide support. Come on, give me a break. Come with me. Watch.” Concerns about Biden’s age have dogged his time in office but they exploded into open view late last month after the first of two scheduled debates with Trump. Onstage in Atlanta, Biden appeared hesitant, confused and physically diminished, struggles aides put down to a cold and jet lag. In comparison, Trump spewed lies virtually unchecked by his opponent or CNN moderators working to rules that precluded instant fact checks. The result was a polling bump for Trump and panic among Democrats. By Monday, nine House Democrats had called for Biden to quit. A reported move towards a similar call in the Senate did not produce a result. Biden insisted his poor debate was down to health issues. “I was feeling so badly before the debate,” he told MSNBC. “They tested me, they thought maybe I had Covid, maybe there was something wrong, an infection or something. They tested me, they gave me those tests. I was clear. So, I had a bad night.” Touting public appearances since the debate, Biden said he was in vigorous health and out meeting voters more than Trump. “I have a neurological test every single day sitting behind his desk and making these decisions,” Biden said. “You know it, they know it. I’m not bad at what I do.” Signaling the size of Biden’s problem, however, the New York Times cited White House visitor logs when it reported that “an expert on Parkinson’s disease” visited “eight times in eight months from last summer through this spring, including at least once for a meeting with President Biden’s physician”. Andrew Bates, a White House spokesperson, told the paper: “A wide variety of specialists … visit the White House complex to treat the thousands of military personnel who work on the grounds.” Bates also said Biden had been seen by a neurologist once a year, finding “no sign of Parkinson’s and he is not being treated for it”. In his open letter to Democrats, the president said he was “firmly committed to staying in this race, to running this race to the end, and to beating Donald Trump”. He also claimed that in “extensive conversations with the leadership of the party, elected officials, rank and file members and most importantly Democratic voters”, he had “heard the concerns that people have – their good faith fears and worries about what is at stake in this election. I am not blind to them. “Believe me, I know better than anyone the responsibility and the burden the nominee of our party carries. I carried it in 2020 when the fate of our nation was at stake.” Biden defeated Trump handily then. But on inauguration day, he was 78 – as old as Trump is now but the oldest man ever to take the presidential oath. On Monday, Biden said: “I wouldn’t be running again if I did not absolutely believe I was the best person to beat Donald Trump.” It is unclear what mechanism could be used for replacing Biden, whether with his vice-president, Kamala Harris, or another candidate. In his letter, Biden pointed to his easy primary win over Dean Phillips, a Minnesota representative who campaigned on the issue of Biden’s age. The president also pointed to the independent Robert F Kennedy Jr, who threatens to take votes in battleground states. “Do we now just say this process didn’t matter?” Biden asked. “That the voters don’t have a say? “I decline to do that. I feel a deep obligation to the faith and the trust the voters of the Democratic party have placed in me … it was their decision to make. Not the press, not the pundits, not the big donors, not any selected group of individuals, no matter how well intentioned. The voters – and the voters alone – decide the nominee. “How can we stand for democracy in our nation if we ignore it in our own party? I cannot do that. I will not do that.” Biden said he had “no doubt” he would beat Trump, touting achievements in office. He also said that in a second term, with a Democratic-controlled Congress, he would restore abortion rights by enshrining them in law, while bringing “real supreme court reform” – an ambitious statement, given a Senate map highly favourable to Republicans. Finally, Biden said he was “standing up for American democracy”. His letter invoked the 6 January 2021 US Capitol attack carried out by Trump supporters, saying his White House predecessor “has proven that he is unfit to ever hold the office of president. We can never allow him anywhere near that office again. And we never will. “We have 42 days to the Democratic convention and 119 days to the general election … it is time to come together, move forward as a unified party, and defeat Donald Trump.”