2020 Democratic presidential candidates become nervous when front-runner Joe Biden begins to speak on the debate stage, and always appear ready for him to commit another public gaffe, said "Fox News Sunday" host Chris Wallace on Thursday.

"A lot of Democrats, when he's talking in those debates, are kind of nervously holding their hands," he said on "The Brian Kilmeade Show" .

"Not that they're necessarily pro-Biden, but they're just thinking, 'I hope he's going to finish this sentence. I hope it's going to end with a period.'"

Wallace said the Ukrainian scandal involving Biden's son, Hunter, and Burisma Holdings has hurt his campaign, but claimed it's his poor debate performances that are hobbling his chances of ascending to the White House next November.

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"With Biden, I think there is a question as to whether he's a good candidate or not," Wallace said earlier in the interview. "And I think there are increasing doubts [about] whether he's up to it. Yes, he's been hurt... by all this Ukraine stuff... but it's just his performance on the stage."

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He also said the current 2020 field of candidates is unappetizing to Democratic voters and has stoked talk of a dark horse entering the race at the eleventh hour, in a last-ditch effort to defeat President Trump and save the party.

Speculation has swirled this week that Hillary Clinton is considering launching a third presidential campaign, as mainstream Democrats remain anxious about the top contenders.

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"Whether it's some of them [being] too far to the left in policies, others I think with their performance ... they're not very happy with the people that are in the race," he added.