Tina Smith, a senator from Mr Phillips’s home state, said: “I know the vice-president well and know her to be exceptional.”
Ms Smith described the vice-president as “always prepared” and “deeply knowledgeable”, adding – in a pointed aside – that she was also “loyal”.
Mr Phillips, 54, launched his long-shot bid against Mr Biden, 81, a month ago, saying the US president’s age and poor polling had become an “emergency”.
He pointed out that Ms Harris’s polling was even worse, in his interview with The Atlantic on Tuesday. “It’s pretty clear that she’s not somebody people have faith in,” Mr Phillips said.
‘I can handle weaklings like Trump’
His late entry to the race has triggered fury from Democrats, who argue that it is too late to block Mr Biden winning the party’s nomination but could severely damage his re-election bid.
Mr Phillips’s bid comes as he faces his own threat of a Democratic primary challenge for his Minnesota district should he seek re-election to Congress.
His wide-ranging interview with The Atlantic was tinged with derision at times, with Mr Phillips at one point claiming he had looked into Donald Trump’s eyes “for the better part of an hour”.
“I know exactly how to handle weaklings like Donald Trump,” Mr Phillips said, but he declined to reveal his strategy. “Why would I give away my special sauce?” he added.
The magazine wryly noted Mr Phillips must first dispatch the incumbent president “before he can douse Trump with his proprietary Dean Sauce”.