Ten Energy Department workers were subpoenaed for their private e-mails late last week as part of a congressional probe of a controversial Obama administration clean energy loan program, and five were served at their offices by a federal marshal.

The move came a month after a House oversight committee released e-mails showing that staffers and contractors frequently discussed federal loans to private energy companies in private e-mails. In one exchange, the head of the loan program warned a staffer how to avoid having private e-mail discussions subpoenaed.

The panel’s ranking Democrat, however, has accused Republicans of trying to intimidate federal workers.

The Obama administration’s clean energy program, including the failure of its showcase firm, Solyndra, has spilled over into the presidential race. GOP challenger Mitt Romney has said that the program is rife with “crony capitalism” and invests taxpayer dollars in companies that helped Obama’s political allies but not middle-class workers.

The White House has said that all loans were awarded based on the merits of the project, not the company’s political connections.

The House Committee on Oversight and Government Reform, led by Chairman Darrell Issa (R-Calif.), on Thursday issued subpoenas for 10 people at the Energy Department’s loan program.

But Rep. Elijah E. Cummings of Maryland, the panel’s ranking Democrat, said, “There was absolutely no reason for this type of intimidation since the department would have accepted the subpoenas by e-mail, as it has done routinely in the past, including with our committee.

“The subpoenas issued last week were particularly unnecessary and unwarranted given that the committee has identified no evidence of improper political favoritism in the department’s loan guarantee program,” Cummings said.

A marshal served five Energy Department employees at their workplace, committee staffers said, because the workers had not responded to a request that they accept the subpoenas electronically. The remaining workers, four contractors and one former employee, had agreed to do so in writing.

Committee spokesman Frederick Hill said it is ironic that Democrats are upset about the subpoenas when they expressed outrage during the Bush administration over the use of private e-mails for official public business. Former chief of staff Karl Rove was accused of trying to keep some of his official correspondence out of public reach by frequently using a private e-mail account, and the Bush White House acknowledged that it had failed to preserve thousands of internal e-mails.

“It’s clear there were widespread and intentional violations of these federal transparency laws in the process of granting controversial loan guarantees,” Hill said. “It’s really shameless that the Cummings letter doesn’t even acknowledge that these clear violations of law occurred.”

Cummings said that the Republican subpoenas appeared to be political posturing during a heated presidential campaign.

“I am deeply concerned that some may view these latest actions by the Committee as abusive, or as an effort to escalate tensions and generate press before the November election, rather than a legitimate attempt to conduct responsible oversight,” the Democrat wrote in a letter to Issa.

The White House refused earlier this year to turn over all Solyndra documents, arguing that it had already provided thousands of records. The solar panel manufacturer received a $535 million federal loan guarantee, and records show that federal staffers complained that the White House was pressuring them to make a decision and that they warned that the company’s future prospects looked weak. Solyndra’s largest investor was the family foundation of George Kaiser, a major Obama fundraising bundler.

A spokesman said that the Energy Department had provided staffers’ private-account e-mails to the committee on Aug. 31, and was baffled by the subpoenas.

“In addition to voluntarily providing more than 1 million pages of documents to Congress, the department has fully complied with the chairman’s requests and provided all of the documents the committee requested from these department officials prior to the committee’s baffling decision to issue subpoenas,” said spokesman Damien LaVera.

“No matter how hard some in Congress are working to distort the record, the facts clearly demonstrate that decisions on loan applications were made on the merits after careful review by career officials and technical experts in the loan program,” he said.

But Republican leaders said the documents turned over on Aug. 31 were only partial and excluded key correspondence between private and public accounts.

In an earlier agreement with the committee, Jonathan Silver, former head of the loan program, agreed to turn over many of his private e-mails discussing DOE loans. The documents show frequent use of private e-mail accounts to discuss companies that the department might fund and the political implications of certain applications. Silver also reached out to top White House officials about some loan decisions.

In one instance, Silver wrote to a staff member on his private e-mail account about a loan connected to Solyndra and asked for another colleague’s private e-mail address. In another document, Silver chastised a staffer about writing about a loan on a government e-mail account and copying another staffer’s private e-mail.

“Don’t ever send an email on doe email with a personal email addresses,” Silver wrote Aug. 21, 2011, from his personal account to a loan program official’s private Gmail account. “That makes them subpoenable,” he wrote.