Former White House Press Secretary Robert Gibbs on Monday blasted House Oversight Committee Chairman Rep. Darrell Issa (R-CA) as "the biggest joke in all of Washington" and said he should apologize for calling current White House Press Secretary Jay Carney a "paid liar."

Speaking to CNN on Sunday, Issa had said that the Internal Revenue Service (IRS) practice of improperly scrutinizing the tax-exempt status of conservative groups had been "ordered from Washington." But under questioning from CNN host Candy Crowley, the California Republican admitted that his evidence was "totally not definitive."

Issa added that Carney was a "paid liar" and "still making up things about what happens and calling this [IRS agent] a local rogue."

"Darrell Issa should call Jay Carney and apologize this morning," Gibbs told MSNBC host Joe Scarborough on Monday. "He's laying out the charge and then saying it might be true. And then he's saying, 'As the investigator, I know where I want to get, now I'm just getting around to proving it.'"

"I mean, it's a stunning thing. It's why five people in this town take Darrell Issa seriously. And it's the surest bet that the Republicans are very much on the verge of overplaying their hand publicly and that the American people will lose interest in their side of this," he added. "To throw around the words liar and perjury as easily as he did is shameful. And if he's got information that these people lied, he should put it out there today."

Co-host Mika Brzezinski pointed out that "Darrell Issa just kind of took it down to a level that makes us all just want to walk away and ignore it."

"It shows how unserious he is about investigating anything," Gibbs agreed. "The notion that he's in charge of -- quote -- Government Oversight might now be the biggest joke in all of Washington."