A Democratic senator on Tuesday joined Republicans in questioning President Barack Obama's firing of the internal watchdog for the federal AmeriCorps program.

Gerald Walpin, the national service agency's inspector general, was dismissed over his handling of an investigation of the mayor of Sacramento, Calif., Kevin Johnson, an Obama supporter during the presidential campaign.

Sen. Claire McCaskill, D-Mo., said the president failed to follow a law she sponsored, which requires that he give Congress 30 days advance notice of an inspector general's dismissal, along with the cause for the firing. Obama merely said he had lost confidence in Walpin.

"Loss of confidence is not a sufficient reason," McCaskill said. "I'm hopeful the White House will provide a more substantive rationale, in writing, as quickly as possible."

Late Tuesday, Obama's special counsel offered some details in a letter, saying Walpin engaged in "trouble and inappropriate conduct."

Norman Eisen sent a letter to McCaskill, Sen. Joseph Lieberman, I-Conn., and Sen. Susan Collins, R-Maine, describing Walpin as "confused, disoriented, unable to answer questions" during a May meeting. Eisen said the behavior led board members to question his capacity to serve.

"Mr. Walpin had become unduly disruptive to agency operations, impairing his effectiveness" and lost the confidence of the board, Eisen wrote.

Last week, Sen. Charles Grassley, R-Iowa, asked for information on any role first lady Michelle Obama's office may have played in the decision.

Grassley requested "any and all records, e-mail, memoranda, documents, communications or other information" related to contacts with officials in the first lady's office.

White House spokesman Josh Earnest said Mrs. Obama played no role in the president's decision to remove Walpin.

Michelle Obama's former chief of staff, Jackie Norris, is expected to join the Corporation for National and Community Service as senior adviser on June 22.

White House counsel Gregory Craig, in a letter to Grassley, cited criticism of Walpin's investigation of Johnson, a former all-star point guard for the Phoenix Suns professional basketball team.

Rep. Darrell Issa, R-Calif., also asked the White House for documents on the firing.

"Despite the requirement to notify Congress in advance of firing ... the White House moved swiftly to sack an investigator who uncovered wrongdoing and abuse by a political ally of the president," said Issa, senior Republican on the House Oversight and Government Reform Committee.

In September 2008, Walpin's office found misuse of federal grants by Johnson and the St. HOPE Academy, a nonprofit education program he founded. Johnson and St. HOPE ultimately agreed to repay half of $847,000 in grants they had received from AmeriCorps between 2004 and 2007.