Top Donald TrumpDonald John TrumpCNN's Don Lemon explains handling of segment after Trump criticism NPR reporter after Pompeo clash: Journalists don't interview government officials to score 'political points' Lawyer says Parnas can't attend Senate trial due to ankle bracelet MORE aide Corey Lewandowski on Sunday defended the presumptive Republican presidential nominee's decision to back out of a debate with Democratic presidential candidate Bernie SandersBernie SandersNew campaign ad goes after Sanders by mentioning heart attack Biden on whether Sanders can unify party as nominee: 'It depends' Steyer rebukes Biden for arguing with supporter he thought was Sanders voter MORE, saying it would be a waste of time because the Vermont senator won't clinch the Democratic nomination. 

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"We're not going to waste our time debating Bernie Sanders when he's not going to be the Democratic nominee," Lewandowski said on "Fox News Sunday." 

"We are squarely focused on defeating [Democratic front-runner] Hillary ClintonHillary Diane Rodham ClintonHill.TV's Krystal Ball: Failure to embrace Sanders as nominee would 'destroy' Democratic Party Clinton says she feels the 'urge' to defeat Trump in 2020 Can Democrats flip the Texas House? Today's result will provide a clue MORE come November," he said. 

The two campaigns had talked about debating each other since Trump said he would be open to it last week on "Jimmy Kimmel Live." 

Trump on Friday said it would be “inappropriate” to hold a debate with Sanders, throwing cold water on an idea that had captivated the political world.

"As much as I want to debate Bernie Sanders — and it would be an easy payday — I will wait to debate the first-place finisher in the Democratic Party, probably Crooked Hillary Clinton, or whoever it may be," Trump said in a statement.

Trump added that it "seems inappropriate" to debate the "second-place finisher."

After hearing the news that Trump had backed away, Sanders said he hopes the billionaire changes his mind and asked: “Well Mr. Trump, what are you afraid of?”

Sanders trails Clinton in the Democratic presidential race but has vowed to stay in the race until the Democratic National Convention in July.

A debate in California would also provide free advertising to Sanders in the biggest primary on the calendar.

— This report was updated at 9:36 a.m.