Democratic presidential candidate Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE expressed surprise after he learned that President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE was thinking about attending Russia's May Day event next year.

After being told the news by a reporter, Biden said "Are you serious?" according to a clip posted Saturday by CNN.

"You're kidding me," the former vice president added. "Are you joking?"

Trump indicated Friday that he would like to accept Russian President Vladimir Putin's invitation to the May Day Parade in Moscow, but said he might not because it is during the presidential campaign season.

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Trump has faced scrutiny over his relationship with Russia, including after he said it was "possible" he would invite Putin to next year's Group of Seven summit.

The Justice Department's investigation into Russian interference in the 2016 election also found that Russia aimed to help Trump win the contest, but did not establish that the Trump campaign conspired with Moscow.