WASHINGTON (AP) — The explosive details of a whistleblower's complaint against President Donald Trump provided Democrats on Thursday with a roadmap for their impeachment inquiry but left Republicans straining under the most serious test yet of their alliance with the White House.

House Speaker Nancy Pelosi said the president engaged in nothing short of a "cover-up" as Democrats turned their laser focus on the Ukraine matter as central to their impeachment probe. Thursday brought striking new revelations about the extent to which the White House sought to shield Trump's call with the new Ukrainian president from public view.

The swiftly moving events caught Republicans off stride and without a unified response. Senate Majority Leader Mitch McConnell stayed silent while other Republicans defended the president and some simply shrugged off what one called the "thin gruel" of the allegations. Yet there was also an undercurrent of concern among Republicans that signaled uncertainty ahead.

"We owe people to take it seriously," Sen. Marco Rubio, R-Fla., a member of the Intelligence Committee, said as he headed toward a closed briefing.

"Right now, I have more questions than answers," he said. "The complaint raises serious allegations, and we need to determine whether they're credible or not."

One certainty was that Congress and the White House are now squaring off for a rare, if not historic, impeachment investigation that will consume both sides and deepen the political divide ahead of the 2020 election.

Pelosi called it a "sad week" in which she, siding with the vast majority of House Democrats, dropped her reluctance to launch an impeachment inquiry of the president.

"This is nothing that we take lightly," she said.

Pelosi read from the whistleblower's declassified complaint, which alleges the White House tried to "lock down" details of Trump's call with Ukrainian President Volodymyr Zelenskiy after he asked him to investigate Democratic presidential candidate Joe Biden.

"This is a cover-up," she said. "The actions taken by this president lifts this into whole new terrain, whole level of concern about his lawlessness."

As the House plunges into an impeachment inquiry, Republican leaders found themselves once again unable to strike a consensus in the face of extraordinary actions coming from the White House that now seem the norm.

McConnell opened the Senate without mentioning the whistleblower's complaint and declined to engage when reporters asked about it in the halls.

The House Republican leader, Rep. Kevin McCarthy of California, defended the White House decision to "lock down" the details of Trump's call by putting all the records of it on a separate computer system.

"Could I see why you'd want to put it on a more secure server?" McCarthy asked. "I think in the world of technology today, yeah, people should secure what's going forward."

The defense of the separate computer system at the White House was striking for Republicans who joined Trump in pursuing information on Hillary Clinton's use of a private server during her time as secretary of state.

Yet the restraint being shown by other Republicans gave nod to the seriousness of the situation and the uncertainty of what is yet to come in the impeachment inquiry.

"There are a lot of questions, absolutely," said Sen. Susan Collins, R-Maine.

Asked about the separate computer system at the White House, Sen. John Cornyn, R-Texas, said, "We're going to have to ask questions about that."

The complaint released Thursday morning alleges that Trump abused the power of his office to "solicit interference from a foreign country" in next year's U.S. election. Trump has denied doing anything wrong.

In the nine pages, the unnamed whistleblower acknowledges not hearing the president's call first-hand but receiving information about it from "multiple U.S. officials."

Much of what the whistleblower recounts from the president's July 25 call tracks with a transcript released Wednesday by the White House.

Sen. Ron Johnson, R-Wis., who made several trips himself to meet with Ukraine's new president, including his inauguration in May, brushed off critics "impugning all kinds of nefarious motives here."

"This has been blown way out of proportion," said Johnson, the chairman of the Homeland Security Committee as well as a leader of the Senate's Ukraine Caucus. He said he talked to Trump before and after those trips and said the president doesn't think he did anything wrong.

"I take what President Trump is saying at face value," Johnson said. Trump, he said, was consistently concerned about corruption in Ukraine and wanted European allies to step up with more foreign aid. "None of this came as a surprise to me."

One Trump ally, Sen. David Perdue, R-Ga., said, "There is absolutely nothing in this phone call that rises to the level of that (impeachment)."

It was a common refrain from other Republicans. Perdue said that from his own talks with Trump, it's clear that he's "moving on."

And several leading Republicans joined Trump in casting doubts about the whistleblower.

Sen. Chuck Grassley, R-Iowa, said, "I wouldn't want to make too quick of a conclusion when you're reading something that somebody heard somebody else say second-hand or third-hand."

As Democrats dive into impeachment proceedings, Pelosi said the information about the president's call "removed all doubt that we should move forward."

The Ukraine question will now become the central focus of the Intelligence Committee headed by Rep. Adam Schiff of California, a former federal prosecutor, and perhaps the House's most effective investigator.

Schiff said the whistleblower "has given us a roadmap for our investigation."

The committee is planning to talk to the whistleblower and probe what role Attorney General William Barr and Trump's personal lawyer, Rudy Giuliani, had in the matter.

Months after the close of former special counsel Robert Mueller's probe of the 2016 election, as well as the House's own investigations into Trump's administration and business dealings, the impeachment probe is now just beginning.

Pelosi said the president "betrayed his oath of office, our national security and the integrity of our elections." She would not put a timeline on the investigation. "We have to have an inquiry to further establish the facts."

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Associated Press writers Alan Fram, Andrew Taylor, Mary Clare Jalonick and Matthew Daly contributed to this report.