Elena Moore

Updated at 3:50 p.m. ET

Four witnesses are testifying in front of the House Intelligence Committee on Tuesday, starting the second week of public hearings in the ongoing impeachment inquiry investigation.

The morning session features Jennifer Williams, a career State Department staffer detailed to work with Vice President Pence's staff, as well as Lt. Col. Alexander Vindman, the Ukraine specialist on the National Security Council. Former U.S. envoy to Ukraine Kurt Volker and former Russia director for the NSC Tim Morrison are testifying in the afternoon.

Impeachment investigators have released the testimony transcript of Tim Morrison, the former top European affairs official on President Trump's National Security Council.

In his Oct. 31 testimony, Morrison confirmed he was on the July 25 phone call between Trump and Ukraine's president. He also said that Trump had indeed sought help from his Ukrainian counterpart, but he argued the conduct wasn't unlawful.

The warnings from national security leaders in Washington have been clear: The danger of foreign election interference isn't going away.

What have the many Democratic presidential hopefuls pledged to do about it?