A Republican senator said Friday he learned from a U.S. ambassador that President Donald Trump was tying military aid for Ukraine to an investigation of the 2016 election. But when the senator asked Trump if he could assure the Ukraine leadership the money would be coming, the president blocked him from carrying that message.

Sen. Ron Johnson of Wisconsin, a leader of the Senate’s Ukraine caucus, made several trips to the Eastern European ally this year after the election of President Volodymyr Zelenskiy. He told the Wall Street Journal that Gordon Sondland, the U.S. Ambassador to the European Union, told him the Ukraine aid, which is part of the country’s defenses against Russia, was being linked to Trump’s desire to have Zelenskiy’s team investigate the 2016 U.S. elections.

Separately, Johnson told reporters in Sheboygan, Wis., that Trump had blocked his suggestion that he carry a message to Ukraine’s president assuring him that U.S. military aid was on the way.

“I was surprised by the president’s reaction and realized we had a sales job to do,” Johnson told reporters....