For many months, President Biden has been rallying global leaders to provide more military aid to Ukraine and pressing Congress to pass a multibillion-dollar aid package to help the country beat back Russian aggression. Former President Donald J. Trump has been undermining that effort, pressing Republicans to thwart it.

But on Wednesday, Mr. Trump tried to flip the script, suggesting that he would do more to protect Ukraine than Mr. Biden, who he said would effectively cede Ukraine as a gift to President Vladimir V. Putin of Russia.

Speaking at a campaign event in North Charleston, S.C., Mr. Trump said that, under a Biden presidency, Mr. Putin is “going to be given everything he wants, including Ukraine. That’s a gift. He’s got a gift.”

Then Mr. Trump — who often positively invokes Mr. Putin as an authoritarian strongman, and who acknowledged in his speech that they got along — doubled down, saying that Mr. Biden “is going to give” Ukraine to Mr. Putin.

Mr. Biden has repeatedly vowed to help Ukraine defend itself for “as long as it takes,” pledging that “our commitment to Ukraine will not weaken.” Mr. Trump, by contrast, has previously said that he would consider letting Russia “take over” parts of Ukraine in a negotiated deal to end the war.

A day earlier, the Senate, in a bipartisan vote, approved an additional $60.1 billion in assistance to Kyiv to help it fight Russia’s invasion, part of a long-debated foreign aid package for Ukraine and Israel.