WASHINGTON — House impeachment investigators on Wednesday summoned John R. Bolton, President Trump’s former national security adviser, and two top White House lawyers to testify next week in their inquiry into Mr. Trump’s pressure campaign on Ukraine, closing in on critical witnesses as they prepare to go public with their investigation.

Mr. Bolton, a fiery foreign policy veteran, could be a marquee player in the House’s month-old impeachment inquiry. His deputies have testified that Mr. Bolton, who left the White House in September amid disagreements with the president, was angry about the efforts to pressure Ukraine to open investigations into Democrats. Rudolph W. Giuliani, the president’s personal lawyer who was leading the charge, he warned, was a “hand grenade who’s going to blow everybody up.”

But his appearance is far from assured. His lawyer said that Mr. Bolton was “not willing to appear voluntarily,” declining to specify what his client would do should he be subpoenaed.

The much-anticipated invitation punctuated another hectic day in Washington, where talk of impeachment and the shadow diplomatic efforts at the heart of the case consumed the capital. A confirmation hearing for John Sullivan, the deputy secretary of state who is Mr. Trump’s nominee to be the ambassador to Russia, was dominated by questions about the Ukraine affair.