Zelensky, in the United States for the United Nations General Assembly, has been presenting U.S. officials with his “victory plan” to end the war against Russia. But his visit has simultaneously antagonized Republicans, whose support is needed for future military assistance.

In an interview with the New Yorker, Zelensky called Trump’s running mate, Sen. J.D. Vance (R-OH), “too radical” over his views on Ukraine while suggesting that Trump’s plan to end the war in a single day was nothing more than a campaign slogan.

“I think it would be advisable for him to stay out of American politics,” Senate Minority Whip John Thune (R-SD) told the Washington Examiner. “They have their differences on some issues, but it’s not his place to litigate that here in the middle of an American election.”

The warning comes ahead of Zelensky’s visit to Capitol Hill on Thursday. He has the support of Senate leadership, including McConnell, but the GOP conference is still stinging over the roughly $60 billion in foreign aid Washington approved earlier this year.

A sizable minority of Senate Republicans voted against the package in April, questioning the value of the funding without a clear off-ramp to the conflict.

Those wounds were reopened on Sunday when Zelensky visited a munitions plant in Scranton, Pennsylvania. The visit was billed as a thank-you to the workers helping with Ukraine’s war effort, but Republicans called the trip “election interference” in a swing state.

“I think that was a monumental blunder for Zelensky to get involved in American politics,” said Sen. John Cornyn (R-TX), an adviser to McConnell. “I mean, it’s just not very smart. Ukraine needs all the friends it can get.”

Both Cornyn and Thune are running to replace McConnell as GOP leader next year.

Thune doubted there is an appetite to give Ukraine additional support in Congress, though he judged that the Biden administration could draw from existing funds in the near term. 

“I doubt it. We just don’t have the bandwidth to deal with it,” Cornyn added in a separate interview.

The Biden administration’s authority to send $6 billion in equipment will expire at the end of the month. 

When asked about Zelensky’s remarks on Wednesday, Vance sidestepped his criticism but said all options must be considered in negotiating an end to the war, presumably including Ukraine ceding some land to Russia.

“He’s going to negotiate an end to the conflict,” Vance said of Trump. “And of course, everything is going to be on the table, but I think nothing is going to be definitively on the table.”

Marisa Schultz contributed to this story.