The ambassador to Ukraine who questioned withholding military aid to the country for political purposes is expected to appear before Congress on Tuesday as part of the impeachment inquiry into President Trump.

William Taylor, a career diplomat, is expected to be a key witness in the investigation into Trump’s phone call with Ukrainian President Volodymyr Zelensky in which he sought an investigation into a political rival, former Vice President Joe Biden.

In a message to other diplomats, Taylor raised the strategy of withholding nearly $400 million in aid to Ukraine.

“I think it’s crazy to withhold security assistance for help with a political campaign,” he wrote to Ambassador to the European Union Gordon Sondland and former US Special Envoy to Ukraine Kurt Volker.

Sondland and Volker have both testified behind closed doors to congressional committees conducting the impeachment probe.

Taylor will testify behind closed doors to the House Intelligence, Oversight and Foreign Affairs committees.

Democrats claim Trump held up the military aid, which Ukraine uses to battle Russian-backed insurgents in the eastern part of the country, to pressure Zelensky to begin the investigation.

Sondland in his testimony last week addressed the message from Taylor and said Trump assured him that no quid pro quo existed.

He said he reached out to the president to ask about Taylor’s concerns.

“I asked the president: ‘What do you want from Ukraine?’ The President responded, ‘Nothing. There is no quid pro quo.’ The President repeated: ‘no quid pro quo’ multiple times,” Sondland said in his opening statement, which was released publicly.