WASHINGTON (AP) — The shifting White House explanation for President Donald Trump's decision to withhold military aid from Ukraine drew alarm Friday from Republicans as the impeachment inquiry brought a new test of their alliance.

Trump, in remarks at the White House, stood by his acting chief of staff, Mick Mulvaney, whose earlier comments undermined the administration's defense in the impeachment probe. Speaking Thursday at a news conference, Mulvaney essentially acknowledged a quid pro quo with Ukraine that Trump has long denied, saying U.S. aid was withheld from Kyiv to push for an investigation of the Democratic National Committee and the 2016 election. He later clarified his remarks.

Trump appeared satisfied with Mulvaney's clarification and the president dismissed the entire House inquiry as "a terrible witch hunt. This is so bad for our country."

But former Ohio Gov. John Kasich, who ran against Trump in the 2016 Republican primary, said he now supports impeaching the president.

Mulvaney's admission, he said, was the "final straw." ''The last 24 hours has really forced me to review all of this," Kasich said on CNN.

In Congress, at least one Republican, Rep. Francis Rooney of Florida, spoke out publicly, telling reporters that he and others were concerned by Mulvaney's remarks. Rooney said he's open to considering all sides in the impeachment inquiry. He also said Mulvaney's comments cannot simply undone by a follow-up statement.

"It's not an Etch-A-Sketch," said Rooney, a former ambassador to the Holy See under President George W. Bush.

"The only thing I can assume is, he meant what he had to say — that there was a quid pro quo on this stuff," he said.

The tumult over Mulvaney's remarks capped a momentous week in the impeachment investigation as the admission, from highest levels of the administration, undercut the White House defense and pushed more evidence into the inquiry.

GOP leaders tried to contain the fallout. But four weeks into the inquiry, the events around Trump's interaction with the Ukraine president, which are are at the heart of impeachment, have upended Washington.

The Energy Secretary, Rick Perry, who has been caught up in the probe, announced his resignation. A beloved House chairman, Rep. Elijah Cummings, D-Md., a leading figure in the investigation, died amid ongoing health challenges.

The march toward an impeachment vote now seems all but inevitable, so much so that the highest-ranking Republican, Senate Majority Leader Mitch McConnell, privately told his GOP colleagues this week to expect action in the House by Thanksgiving with a Senate trial by Christmas.

Speaker Nancy Pelosi has given no timeline for conclusion but wants the inquiry completed "expeditiously." She said Thursday that facts of the investigation will determine next steps.

"The timeline will depend on the truth line," she told reporters.

This week's hours of back-to-back closed-door hearings from diplomats and former top aides appeared to be providing investigators with a remarkably consistent account of the run-up and aftermath of Trump's call with Ukraine President Volodymy Zelenskiy.

In that July call, Trump asked the newly elected Zelenskiy for a "favor" in investigating the Democratic National Committee's email situation, which was central to the 2016 election, as well as a Ukraine gas company, Burisma, linked to the family of Trump's 2020 Democratic rival, Joe Biden, according to a rough transcript of the phone conversation released by the White House.

Republican leaders tried to align with Trump Friday, amid their own mixed messages as House Democrats, who already issued a subpoena to Mulvaney for documents, now want to hear directly from him.

Rep. Kevin McCarthy, the House GOP leader, cited Mulvaney's clarification as evidence that there was no quid pro quo. He said witnesses have also testified similarly behind closed doors in the impeachment inquiry.

"We've been very clear," McCarthy said. "There was no quid pro quo."

Lawmakers involved in the three House committees conducting the investigation want to hear more next week, which promises another packed schedule of witnesses appearing behind closed doors.

Republicans want the interviews made open to the public, including releasing transcripts.

Democrats in the probe being led by Rep. Adam Schiff, chairman of the Intelligence Committee, are keeping the proceedings closed for now, partly to prevent witnesses from comparing notes.

Three House committees investigating impeachment have tentatively scheduled several closed-door interviews next week, including one with Bill Taylor, the current top official at the U.S. Embassy in Ukraine.

Taylor's interview, scheduled for Tuesday, is significant because he was among the diplomats on a text message string during the time around the July phone call. He raised a red flag and said it was "crazy" to withhold the military aid for a political investigation.

It's unclear whether all the witnesses will appear, given that the White House is opposing the inquiry and trying to block officials from testifying.

The schedule includes a mix of State Department officials and White House aides.