Sen. Charles E. Grassley (Iowa) blocked two of President Trump’s nominees Thursday in a rare move by a Republican senator to demand accountability from the president over his recent firings of several federal watchdogs.

Grassley, a longtime advocate for inspectors general, announced Thursday afternoon that he is blocking the nominations of Christopher Miller to head the National Counterterrorism Center and Marshall Billingslea to be the State Department’s undersecretary for arms control and international security.

Miller is an Army Special Forces veteran serving as a Pentagon special operations and counterterrorism official, while Billingslea is assistant secretary for terrorist financing at the Treasury Department.

Grassley said he will not allow consideration of Miller’s nomination to proceed until the White House provides answers on Trump’s firing in April of intelligence community inspector general Michael Atkinson.

Billingslea’s nomination, Grassley said, cannot proceed until Trump explains why he terminated State Department inspector general Steve Linick last month. Trump abruptly fired Linick at what both he and Secretary of State Mike Pompeo said was Pompeo’s request, although the details remain unclear.

The White House did not immediately respond to a request for comment.

Inspectors general serve as internal government watchdogs conducting oversight of federal agencies — and although they technically are political appointees, their independence has long been protected.

During the coronavirus pandemic, Trump has fired or pushed aside a total of five inspectors general. In addition to Atkinson and Linick, he has pushed out Glenn Fine, chairman of the federal panel Congress created to oversee his administration’s management of the government’s $2 trillion coronavirus stimulus package. He removed Christi Grimm as principal deputy inspector general for the Department of Health and Human Services, after Grimm’s office criticized the administration’s response to the pandemic. And he replaced the acting inspector general at the Department of Transportation.

Democrats have denounced the firings as a retaliatory purge and an effort by Trump to avoid accountability. Several lawmakers, including Grassley, have sought answers from Trump beyond his citing of a general lack of confidence in the watchdogs.

In a statement explaining his move Thursday, Grassley said he does not dispute Trump’s authority to fire the inspectors general, but he argued that “without sufficient explanation, the American people will be left speculating whether political or self-interests are to blame.”

“Though the Constitution gives the president the authority to manage executive branch personnel, Congress has made it clear that should the president find reason to remove an inspector general, there ought to be a good reason for it,” Grassley said. “The White House’s response failed to address this requirement, which Congress clearly stated in statute and accompanying reports.”

Grassley had previously written to Trump on the issue, arguing that his broad declarations of a lack of confidence were “not sufficient” to fulfill the requirements of the 2008 Inspector General Reform Act.

Atkinson said in April that he believes he was fired for having properly handled a whistleblower complaint that became a centerpiece of the case for Trump’s impeachment.

Linick, meanwhile, appeared before Congress on Wednesday in a virtual, closed-door session. But lawmakers of both parties said they came away with little better sense of the specifics surrounding his termination.

Pompeo has previously said that Linick pursued investigations of policies he disagreed with, that his office was responsible for leaks, and that he was not supportive of the secretary’s “ethos statement” on department behavior.

According to a person familiar with Wednesday’s congressional interview, who spoke on the condition of anonymity about the closed-door session, Linick confirmed that his office was looking into allegations that Pompeo and his wife asked personnel to do personal errands for them, as well as the administration’s bypassing of congressional approval for arms sales to Saudi Arabia, but he declined to speculate as to whether either of those matters had prompted his firing.

Karen DeYoung, Seung Min Kim, Ellen Nakashima and Philip Rucker contributed to this report.