Former U.S. Rep. Beto O’Rourke on Tuesday said impeachment proceedings should begin against President Trump because of the administration’s refusal to comply with “any subpoena.”

Speaking at a CNN town hall, the 2020 contender from El Paso, Texas, said while it’s “not something that I take lightly,” he believes there are no other options than to start the impeachment process.

“It’s an incredibly serious, sober decision that we should make as a country. Really the last resort when every other option has failed us,” O’Rourke, a Democrat, said. He added that it’s because Trump won’t respond to subpoenas and overall because he won’t allow witnesses to testify to Congress “so we can find out what happened to this great democracy in 2016.”

“A president invited the involvement of a foreign power in this democracy in 2016, and then did everything in his power to obstruct the investigation into what has happened,” the former congressman said.