WASHINGTON -- Although legal experts warned at the time that little would come of Rep. Darrell Issa's (R-Calif.) attempt to prosecute former IRS official Lois Lerner for contempt of Congress, Republicans on Issa's Oversight and Government Reform Committee were infuriated to learn Thursday that a key obstacle is a Reagan administration legal opinion.

Issa's committee and then the full House voted to hold Lerner in contempt because she twice asserted her Fifth Amendment right in refusing to testify about her role in the IRS's botched screening of political nonprofits. She led the unit that oversees whether such groups get tax breaks, and was in charge when an inspector general found the IRS used "inappropriate" terms that largely singled out conservative groups.

When Congress finds a person in contempt, the matter is referred to federal prosecutors to be brought before a grand jury.

Legal experts advised against taking the step, and one of them, Gregory Gilchrist, told HuffPost at the time that it was unlikely a prosecutor would take up such a case, even though federal law spells out that pathway.

The reason, he said, is that not only were the facts in the case weak, but courts have historically given prosecutors wide leeway in deciding whether to bring cases.

"I just can't imagine that they would proceed with the case," Gilchrist said. "Unless the U.S. attorney takes a different view of the merits than I do, which I don't expect he will, I don't see any way this ends up in an actual charge."

At Thursday's hearing, several Republicans demanded that Deputy Attorney General James Cole explain why prosecutors had not already moved forward.

"This Congress held Lois Lerner in contempt, geez, almost nine weeks ago," Rep. Ron DeSantis (R-Fla.) said, citing the procedure that's spelled out in law that says the prosecutor's duty "shall be to bring the matter before the grand jury."

But Cole noted that the prosecutor still gets to decide.

"My understanding of the law is that it does not strip the U.S. attorney of the normal discretion that the U.S. attorney has," Cole said. "He proceeds with the case if he believes it is appropriate to do so."

His answer infuriated Republicans, especially Issa, who demanded proof.

"If you think that's discretionary, would you please give that back to us in a legal opinion so that we can change the law to make it clear you're wrong," Issa said.

Issa's Democratic counterpart on the committee, Rep. Elijah Cummings (Md.) was happy to find that opinion himself, written by conservative lawyer Theodore Olson when he worked for President Ronald Reagan's Office of Legal Counsel in 1984.

"What it says is, 'We believe Congress may not direct the executive to prosecute a particular individual without leaving any discretion to the executive to determine whether a violation of the law has occurred.' That's what the opinion says -- a 1984 opinion dated May 30," Cummings said. "This was a contempt citation coming from Congress that he was talking about."

The Obama administration wouldn't be the first to rely on that opinion. The White House also cited it under Bill Clinton and George W. Bush. And although Issa described it as a "new" assertion in the hearing, his own committee heard it in 2012 when Congress voted to hold the attorney general himself in contempt. Indeed, the letter asserting it was written by Cole, and Issa was CC'd.

Watch the hearing above.