The investigation is an unusually bold move for Ukraine’s government, which has tried to stay out of the impeachment debate and needs Washington’s help in its conflict with Russia.

It centers on text messages that suggest Robert Hyde, an obscure Republican candidate for Congress, secretly tracked the movements of the diplomat, Marie Yovanovitch, whom Mr. Trump later removed from Ukraine.

Background: Lev Parnas, the Soviet-born businessman who exchanged the messages with Mr. Hyde, turned them over to congressional Democrats scrutinizing Mr. Trump’s campaign to pressure Ukraine to investigate his political rivals. Mr. Parnas had worked with Mr. Trump’s personal lawyer, Rudolph Giuliani, to oust Ms. Yovanovitch, whom the president’s allies considered an impediment.

Responses: In separate television interviews on Wednesday, Mr. Parnas said he had not taken Mr. Hyde seriously, and Mr. Hyde said the messages were “colorful texts” in which he was “joking around.”

Related: A nonpartisan U.S. government watchdog said the Trump administration had violated American law by withholding nearly $400 million in congressionally allocated security aid to Ukraine — a decision at the heart of the impeachment case.