Republican lawmakers are battening down the hatches on Capitol Hill as they face a barrage of questions about President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE’s communications with Ukraine, and whether the White House tried to cover it up.

While Republicans in the House have broadly backed Trump with a few exceptions, GOP lawmakers in the Senate on Thursday pulled back a bit on their defense in the wake of the release of a complaint that alleges Trump sought to enlist Ukraine in a politically motivated attack on former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE.

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After offering a stouter defense of Trump on Wednesday after the White House released a rough transcript of Trump’s July 25 call with Ukrainian President Volodymyr Zelensky, GOP lawmakers were more tight-lipped Thursday, saying they wanted to study the nine-page document.

Sen. Ted Cruz Rafael (Ted) Edward CruzCrenshaw looms large as Democrats look to flip Texas House seat SCOTUS confirmation in the last month of a close election? Ugly The Hill's Morning Report - Sponsored by Facebook - Trump previews SCOTUS nominee as 'totally brilliant' MORE (R-Texas) referred questions about the whistleblower’s complaint to his office.

Sen. Lamar Alexander Andrew (Lamar) Lamar AlexanderPelosi urges early voting to counter GOP's high court gambit: 'There has to be a price to pay' Graham: GOP has votes to confirm Trump's Supreme Court nominee before the election The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy MORE (R-Tenn.), a Senate institutionalist who voted on Wednesday for a resolution disapproving of Trump’s efforts to repurpose military funding to pay for a border wall, said he wants to wait and see what else develops.

“I’m waiting to see what the Intelligence Committee concludes about its investigation before I issue any judgments,” he said.

Senate Republican Whip John Thune John Randolph ThuneHouse to vote on resolution affirming peaceful transition of power Trump dumbfounds GOP with latest unforced error Senate passes resolution reaffirming commitment to peaceful transition of power MORE (S.D.), who on Wednesday said he wouldn’t have brought up the subject of investigating Biden on a call with a foreign leader, on Thursday ducked out of Capitol en route to a two-week recess, telling reporters he still needed to read the complaint.

One Republican senator said there’s trepidation about what may emerge, particularly over what steps senior White House officials may have taken to keep the transcript of Trump and Zelensky’s call from emerging.

An unnamed whistleblower in a letter to Senate Intelligence Committee Chairman Richard Burr Richard Mauze BurrHillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns Bipartisan representatives demand answers on expired surveillance programs Rep. Mark Walker says he's been contacted about Liberty University vacancy MORE (R-N.C.) and House Intelligence Committee Chairman Adam Schiff Adam Bennett SchiffSchiff to subpoena top DHS official, alleges whistleblower deposition is being stonewalled Schiff claims DHS is blocking whistleblower's access to records before testimony GOP lawmakers distance themselves from Trump comments on transfer of power MORE (D-Calif.) said that senior White House officials “intervened to ‘lock down’ all records of the phone call, especially the official word-for-word transcript.”

These were actions that the whistleblower said “underscored to me that White House officials understood the gravity of what had transpired in the call.”

As things stand, the GOP senator said it’s highly unlikely Senate Republicans will vote for articles of impeachment. But the source said there’s some uneasiness about what else might emerge, especially as investigators delve into the question of whether Trump’s call was improperly designated as classified to keep it from becoming public.

“We’ll see where this takes us. It’s hard to imagine this turns out to be anything but a party-line vote,” the lawmaker said. “But we may come across some stuff embroiled in this question of how the transcript was marked classified."

“Was it improperly marked classified?” said the lawmaker. “Other things could come out.”

A senior Republican aide suggested that some GOP lawmakers are finding it tough to defend Trump’s behavior as laid out in the complaint.

“It’s a tough time to be answering questions right now,” the aide said.

One key development late Wednesday was the emergence of Sen. Ben Sasse Benjamin (Ben) Eric SasseWhy a backdoor to encrypted data is detrimental to cybersecurity and data integrity McEnany says Trump will accept result of 'free and fair election' McConnell pushes back on Trump: 'There will be an orderly transition' MORE (R-Neb.), who has received Trump’s endorsement for reelection, as a critical voice.

He warned that “Republicans ought not to be rushing to circle the wagons to say ‘there’s no there there’ when there’s obviously lots that’s very troubling there,” he told a small group of reporters.

In the House, Intelligence Committee member Rep. Michael Turner (R-Ohio) said Trump’s conversation with Zelensky was “not OK.”

“And I think it's disappointing to the American public when they read the transcript,” he said during an Intelligence Committee hearing on Thursday.

Retiring Rep. Will Hurd William Ballard HurdHillicon Valley: Oracle confirms deal with TikTok to be 'trusted technology provider' | QAnon spreads across globe, shadowing COVID-19 | VA hit by data breach impacting 46,000 veterans House approves bill to secure internet-connected federal devices against cyber threats House Democrats' campaign arm reserves .6M in ads in competitive districts MORE (R-Texas), a former undercover CIA officer, called for a full investigation.

“There is a lot in the whistleblower complaint that is concerning. We need to fully investigate all of the allegations addressed in the letter, and the first step is to talk to the whistleblower,” Hurd tweeted on Thursday.

House GOP leaders presented an aggressive defense of Trump, with Minority Whip Steve Scalise Stephen (Steve) Joseph ScaliseGinsburg becomes the first woman to lie in state in the Capitol House GOP slated to unveil agenda ahead of election House panel details 'serious' concerns around Florida, Georgia, Texas, Wisconsin elections MORE (R-La.) saying Speaker Nancy Pelosi Nancy PelosiDemocratic senator to party: 'A little message discipline wouldn't kill us' Overnight Health Care: New wave of COVID-19 cases builds in US | Florida to lift all coronavirus restrictions on restaurants, bars | Trump stirs questions with 0 drug coupon plan Overnight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds MORE (D-Calif.) owed “an apology” to the American public for launching an impeachment inquiry.

House Minority Leader Kevin McCarthy Kevin Owen McCarthyHouse to vote on resolution affirming peaceful transition of power Ginsburg becomes the first woman to lie in state in the Capitol McCarthy says there will be a peaceful transition if Biden wins MORE (R-Calif.) argued that Trump didn’t push Zelensky to investigate Biden but only to cooperate in Attorney General William Barr Bill BarrFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Why a backdoor to encrypted data is detrimental to cybersecurity and data integrity FBI official who worked with Mueller raised doubts about Russia investigation MORE’s investigation of foreign interference in the 2016 election.

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“That is lawful, for us to look for another government to actually participate in an open investigation of what happened in 2016 because we want to make sure that never happens again,” he said.

Some of Trump’s loudest allies in the Senate also offered a vigorous defense.

Senate Judiciary Committee Chairman Lindsey Graham Lindsey Olin GrahamLincoln Project mocks Lindsey Graham's fundraising lag with Sarah McLachlan-themed video The Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election Trump dumbfounds GOP with latest unforced error MORE (R-S.C.) argued the transcript of the call between Trump and Zelensky showed he didn’t do anything improper. He also dismissed the findings of former special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE’s report as grounds for impeachment.

But Sen. Susan Collins Susan Margaret CollinsDemocratic senator to party: 'A little message discipline wouldn't kill us' Poll: 57 percent of Americans think next president, Senate should fill Ginsburg vacancy On The Trail: Making sense of this week's polling tsunami MORE (R-Maine), a pivotal swing vote in any Senate impeachment trial who also faces a tough reelection battle, said she needed to know more facts before passing judgment on whether the transcript was improperly marked classified.

“I don’t know the facts of what they did,” she said when asked about the White House handling of the transcript.

Sen. Ron Johnson Ronald (Ron) Harold JohnsonThe Hill's Morning Report - Sponsored by Facebook - Trump previews SCOTUS nominee as 'totally brilliant' The Hill's 12:30 Report: Ginsburg lies in repose CHC leaders urge Senate to oppose Chad Wolf nomination MORE (R-Wis.), one of Trump’s staunchest defenders, initially told a reporter Thursday that he was concerned about claims that the White House tried to lock down the transcript.

But Johnson later told reporters that once he found out the White House released the full transcript of Trump’s conversation with Zelensky and were not holding back a more detailed account, his concerns were abated.

But the GOP senator who requested anonymity cautioned while it may turn out that White House officials acted improperly in classifying the transcript, the offense was more akin to a modest offsides penalty in football than a flagrant personal foul.

Many Republican senators on Thursday opted for what they saw as the safest route: keeping their mouths shut instead of defending the president.

GOP senators’ standard reply to questions about the allegations in the whistleblower’s complaint released Thursday morning was to say they were still studying it.

Sen. Lisa Murkowski Lisa Ann MurkowskiDemocratic senator to party: 'A little message discipline wouldn't kill us' Overnight Energy: Trump officials finalize plan to open up protected areas of Tongass to logging | Feds say offshore testing for oil can proceed despite drilling moratorium | Dems question EPA's postponement of inequality training Poll: 57 percent of Americans think next president, Senate should fill Ginsburg vacancy MORE (R-Alaska), another potential swing vote, said she wasn’t going to comment until she read the whistleblower’s complaint carefully.

“I’m going to do that one next — on my reading list,” she said.