WASHINGTON — Some of President Trump’s staunchest defenders in the Senate have twisted themselves into contortions to avoid becoming enmeshed in the impeachment inquiry into his pressure campaign on Ukraine.

Then there is Senator Ron Johnson.

Long an outspoken advocate of Ukraine, Mr. Johnson, a manufacturing baron from northeastern Wisconsin elected in 2010 on the Tea Party wave, has landed squarely in the middle of the impeachment inquiry. He is now emerging as one of Mr. Trump’s most vocal congressional allies.

The outsize role Mr. Johnson has played in Mr. Trump’s Ukraine policy was illustrated in an 11-page slash-and-burn letter he released on Monday. In that telling, he appeared over and over as a supporting character in a series of events House investigators are scrutinizing alongside a number of witnesses who will testify this week.

He was there in Kyiv, the capital of Ukraine, and conveyed to the country’s president, Volodymyr Zelensky, that he had tried but failed to persuade Mr. Trump to release the aid. He was there in the Oval Office next to two of the inquiry’s key witnesses to brief Mr. Trump about their meeting with Mr. Zelensky after his inauguration. He was there to personally confront Mr. Trump about allegations he was withholding military aid to Ukraine in exchange for a guarantee the nation would investigate his political rivals.