WASHINGTON — In riveting testimony, a former national security official declared Thursday that a U.S. ambassador carried out a controversial “domestic political errand” for Donald Trump on Ukraine, an allegation undercutting a main line of the president’s defense in the impeachment inquiry.

Fiona Hill told House investigators she came to realize Ambassador Gordon Sondland wasn’t simply operating outside official diplomatic channels, as she and others suspected, but carrying out instructions from Trump.

“He was being involved in a domestic political errand, and we were being involved in national security foreign policy,” she testified, “and those two things had just diverged.”

Hill’s comment followed a blistering back-and-forth during questioning from Republicans at the House hearing.

Testimony from Hill and David Holmes, a State Department adviser in Kyiv, capped an intense week in the historic inquiry and reinforced the central complaint: that Trump used foreign policy for political aims, setting off alarms across the U.S. national security and foreign policy apparatus.

Democrats allege Trump was relying on the discredited idea that Ukraine rather than Russia interfered in the 2016 U.S. election as he sought investigations in return for two things: U.S. military aid that Ukraine needed to fend off Russian aggression, and a White House visit the new Ukrainian president wanted that would demonstrate his backing from the West.

One by one, Hill, a Russia expert at the White House’s National Security Council until this summer, took on Trump’s defenses.

She and Holmes both told House investigators it was abundantly clear Trump’s lawyer Rudy Giuliani was pursuing political investigations of Democrats and Joe Biden in Ukraine.

“He was clearly pushing forward issues and ideas that would, you know, probably come back to haunt us and in fact,” Hill testified. “I think that’s where we are today.”

And Hill stood up for Lt. Col. Alexander Vindman, the Army officer who testified earlier and whom Trump’s allies tried to discredit.” He remains at the White House National Security Council.

At one point, Republicans interjected, trying to cut off Hill’s response as she flipped the script during the afternoon of questioning. The GOP lawmakers had been trying to highlight her differences with Sondland, the ambassador to the European Union who delivered damaging testimony Wednesday about what he said was Trump’s “quid pro quo” pursuit of the political investigations.

The Republican lawmakers eventually wound down their questions but continued with mini-speeches decrying the impeachment effort. Democrats, in turn, criticized Trump’s actions.