As support for impeachment swelled in the House of Representatives on Tuesday, several Democrats in the Senate, including Minority Whip Dick Durbin (Ill.), said they also support opening an inquiry into President Donald Trump.

“I think the House has to initiate an impeachment inquiry as a means of securing the documents and witnesses necessary to understand the whole story, and particularly the latest chapter involving Ukraine,” Durbin, the No. 2 Senate Democrat, told reporters on Tuesday.

The same day, Sens. Brian Schatz (D-Hawaii), Chris Murphy (D-Conn.), Richard Blumenthal (D-Conn.) and Tina Smith (D-Minn.) also called for the House to begin impeachment proceedings, bringing the total number of Senate Democrats who support doing so to 17.

What prompted the move toward impeachment among Senate Democrats, who are more reticent on the matter compared to their House colleagues, is news that Trump apparently asked the head of Ukraine to investigate a political rival, former Vice President Joe Biden, and his family. The Trump administration reportedly withheld a promised $250 million worth of military assistance from Ukraine a week prior to the president’s call with the Ukrainian leader.