Mr Obama, who has generally avoided attacking his successor directly, told a US politics podcast Mr Trump was attempting to "discourage people from voting" by opposing additional funds for the US Postal Service (USPS), which is expected to face a surge of postal votes this November due to coronavirus.
“What we’ve seen in a way that is unique to modern political history is a president who is explicit in trying to discourage people from voting," Mr Obama told his campaign manager David Plouffe.
"What we’ve never seen before is a president say, 'I’m going to try to actively kneecap the postal service'."
Mr Obama's intervention came the same day the US was accused of an "historic humiliation" at the United Nations Security Council, after it failed rejected a US resolution to extend an arms embargo on Iran.
"This is an historic humiliation for the United States. We have never lost a vote this badly; never been so isolated on the global stage. Epic diplomatic malpractice," Joe Cirincione, a national security expert and former official on Capitol Hill, said on Twitter.
Protesters gathered outside the apartment building of the president's recently appointed postmaster general, who has been roundly condemned for sweeping cuts emboldened by the president's threats to vote-by-mail, following reports that the president and First Lady Melania Trump requested mail-in ballots in Florida.
A national outcry against cuts to the USPS was also amplified by musician Taylor Swift, who accused the president of choosing to "blatantly cheat and put millions of Americans' lives at risk in an effort to hold on to power."
"Donald Trump's ineffective leadership gravely worsened the crisis that we are in and he is now taking advantage of it to subvert and destroy our right to vote and vote safely," she said. "Request a ballot early. Vote early."
At a rambling news conference at his private golf club in New Jersey before a meeting with his supporters, the president continued to blame Democratic lawmakers while arguing against additional funding for the Postal Service as it braces for a surge in absentee ballot requests while enduring cuts to personnel and the removal of critically needed mail sorting machines.
He claimed Democrats – seeking funds dedicated to supporting election efforts – want "a trillion dollars to hand out as welfare."
The president also refused, again, to say whether he believes Kamala Harris is eligible to serve as vice president, following his campaign's promotion of an article's argument that has been widely condemned as racist and legally invalid. The president said he would not be "pursuing" those claims.
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