Washington (CNN) Republicans expressed relief on Tuesday when President Donald Trump announced Mark Esper, who currently serves as Secretary of the Army, would be the new acting Secretary of Defense after the dramatic implosion of Patrick Shanahan's nomination .

Esper is replacing Shanahan to lead the largest agency in the federal government as the administration faces multiple foreign policy challenges across the globe.

In multiple interviews and press availabilities, GOP lawmakers, aides, and defense experts voiced confidence in Esper, calling him "solid" and "qualified." Though his seven years working for the Defense contractor Raytheon and his role helping to implement some of Trump's more controversial policies are likely to come under scrutiny from Democrats, Republicans are so far pleased with the Esper pick.

The announcement came after a gut check by the President, who called allies around Washington Tuesday morning to confirm they approved of Esper before making the announcement about the pick and Shanahan's withdrawal on Twitter, according to two sources close to the administration.

GOP enthusiasm toward Esper contrasted to the tepid response that many defense-minded Republicans had to Shanahan's elevation to acting secretary in January and Trump's announcement that he was being nominated for the permanent role in May. Some voiced concerns that Shanahan, a career Boeing employee with no military experience, was unqualified for the job and could face hurdles in his confirmation.

Perhaps most importantly, Esper has the backing of Trump's top two national security officials, Secretary of State Mike Pompeo and National Security Adviser John Bolton.

Taking on a big job

Esper will take on the acting secretary role having already helped navigate Trump's demands for military support on the southern border to bolster his efforts to curtail immigration from Central America.

His first and most pressing task will be overseeing an expanding presence in the Middle East as tensions with Iran rise and it appears the President and key members of his national security team are at odds over how to respond to Tehran.

To that end, many of the Republicans praising him on Tuesday also expressed hope and an expectation that Trump would nominate Esper, the Secretary of the Army since 2017, to lead the Pentagon permanently and provide some much needed stability to the Defense Department, which has been without a permanent leader since James Mattis resigned in December.

Pompeo graduated from the United States Military Academy alongside Esper in 1986. A source with knowledge told CNN that Pompeo is "very happy" with Esper's selection and considers him a friend of 37 years.

Esper also has a good relationship with Bolton, according to a source familiar with both men. While the Defense Department and National Security Council have often been in conflict in previous years -- including during James Mattis' tenure as Trump's Secretary of Defense -- the tensions between the two offices have lowered in recent months and that would likely continue under Esper, said the source.

"This is not the Mattis Defense Department," said the source. "This is a different era than it was and it will continue to be a different era."

A "sparkling resume"

A former senior executive at major defense contractor Raytheon, Esper has also worked as a staffer on defense and national security issues in both Houses of Congress -- experience which has bolstered his relatively good relationship with lawmakers.

Esper was an Army Ranger and served in combat in the Gulf War. He also has master's degree from Harvard and a PhD from George Washington University, culminating in what a senior Republican aide on the Senate Armed Services Committee told CNN was a "sparkling resume."

"Esper would be a very smart pick for the White House," the aide said. "He would have a very smooth confirmation. He's the most qualified in the Department."

Two more Republican sources told CNN that Esper was likely to be Trump's permanent pick. "I wouldn't be surprised if he gets the formal nomination," said one Republican.

Oklahoma Republican Sen. James Inhofe, chairman of the Senate Armed Services Committee, will be a key player in any nomination of Esper and would oversee a potential confirmation hearing. Inhofe had been getting impatient over the lack of a permanent secretary at the Pentagon and expressed confidence in Esper as a potential pick.

"There won't be anything out there that I can think of that will create a problem for him," Inhofe told reporters on Tuesday. "Of course, there was something that did create a problem for Shanahan."

Inhofe told reporters that he's known Esper "for a long time," and that he's viewed first-hand how Esper handles troops in the field. "He does an exceptionally good job," he said.

Republican Sen. Lindsey Graham of South Carolina praised the choice and said Esper was "solid on all the issues that I care about." He added that the naming of Esper would provide more stability at the Pentagon.

Lobbying and the border wall

Esper is unlikely to escape scrutiny, either as acting secretary or if he is nominated for the main job. Citizens for Responsibility and Ethics in Washington -- a watchdog group which raised concerns about Shanahan's Boeing links -- released a statement Tuesday noting the potential for conflicts with Esper's previous lobbying work at Raytheon.

"Already this week, Raytheon has won multiple government contracts worth hundreds of millions of dollars," said CREW's executive director, Noah Bookbinder. "While Esper may not have had sway over these types of deals as Secretary of the Army, as acting Secretary of Defense he will have potential influence over such deals, as well as over the controversial proposed merger of Raytheon and UTC to become the second largest defense company in the US. His ethics agreement -- and his ability to follow it -- will be something we will be watching closely."

And in his current position as Army secretary, Esper has faced Democratic criticism over the Trump administration's proposed use of military funds to build a border wall. He was tasked with identifying money for low-priority Army construction projects that could be re-purposed for the wall.

At a March hearing of the Senate Appropriations Committee's defense subcommittee, Democrats pressed Esper over the President's plan to redirect $1 billion of the Army's budget toward the wall.

Esper defended what Washington Democratic Sen. Patty Murray called a "raiding" of the Army budget as funds that were part of "surplus personnel money." He claimed the use of those funds would not affect military readiness.

Potential nomination hiccups

If the President nominated Esper to be the permanent secretary, there could be some legal complications.

The law governing executive branch vacancies would only allow Esper to serve as acting Defense secretary through July 30 -- 210 days after Mattis resigned from his Senate-confirmed post. (Shanahan was serving as acting secretary under a separate statute.)

The President could nominate someone else as permanent Defense secretary before the end of July, which would allow Esper to continue serving in an acting role for another 210 days or until the Senate confirmed the new nominee. But if Trump nominates Esper, the law requires that he relinquish his role as acting secretary under the Federal Vacancies Reform Act.

A Republican aide on the Senate Armed Services Committee tells CNN that if Esper is nominated, the committee's counsel believes Esper would have to stop being acting in order to go through a formal nomination process.

Correction: This article has been updated to correctly describe Esper's postgraduate education.