Mr. Yanukovych had hired a lobbying firm co-founded by Paul Manafort, now Mr. Trump’s campaign manager, to improve his image in the West and avoid punishment for veering toward Russia.

The annexation of Crimea in 2014 was seen as both a power grab and a land grab by Mr. Putin. It was condemned by the United States and its European allies, which all issued sanctions. Since then, Russian troops, often out of uniform, have been seen, and sometimes killed, in Russian-speaking parts of Ukraine where a pro-Russia insurgency has fought the current Ukrainian government.

Republicans in Congress have long pressed for more assistance to Ukraine to push back against Mr. Putin, including lethal aid. But all references to giving lethal aid to the Ukrainian government were kept out of the party platform.

In early July, a delegate offered a platform amendment to support lethal aid. A delegate for Senator Ted Cruz from Texas, Diana Denman, said in an interview that she had pushed for inclusion of the language. But Ms. Denman said her amendment, as proposed, was never voted on because two men who were observing the panel’s deliberations moved to table the amendment, and suggested that it be discussed later.

“They openly said they were hired by the Trump campaign and worked for Mr. Trump,” Ms. Denman said, adding that she did not recall their names. In the final version of the Republican platform, the words about weapons were dropped and replaced by the term “appropriate assistance.”

Mr. Trump acknowledged in the ABC interview that the language had been watered down, but he said he had nothing to do with it. (Mr. Manafort has also said he was unaware of the matter.)

“I wasn’t involved in that,” Mr. Trump said. “Honestly, I was not involved.” But he acknowledged that his supporters were. “They softened it, I heard, but I was not involved,” he said.