By EVAN PEREZ

WASHINGTON—House lawmakers are set to vote Thursday on whether to hold Attorney General Eric Holder in contempt of Congress in a dispute that began over a botched federal gun-trafficking probe but has become more of a test of political wills.
If approved, as expected, the vote would mark the first such contempt citation against a sitting attorney general. It would likely set up another conflict if the Justice Department, as expected, effectively dismisses the congressional vote and refuses to prosecute the attorney general.
The contempt dispute centers on Justice Department documents sought by Republican lawmakers investigating a bungled gun-trafficking operation called Fast and Furious. Mr. Holder has refused to provide documents that reflect how the department reacted to questions about the operation from Congress last year. Last week, President Barack Obama asserted executive privilege over the documents, after Republicans and Mr. Holder failed to reach a deal to avert the contempt vote in exchange for turning over limited number of documents.
Fast and Furious was a 2009-10 operation run by Arizona-based agents of the Bureau of Alcohol, Tobacco, Firearms and Explosives, aimed at building a case against suspected smugglers of firearms to Mexico. The agents used a tactic called "gun-walking" to allow suspected smugglers to buy about 2,000 firearms, without intercepting the weapons. Some have since turned up at crime scenes on both sides of the border, including at a December 2010 shootout that killed a U.S. border agent.
Rep. Darrell Issa (R., Calif.), chairman of the House Oversight and Government Reform Committee, and Sen. Charles Grassley (R., Iowa) have led the congressional investigation, holding hearings last year with ATF agents who testified about internal dissent over the gun-walking tactics. The lawmakers say the documents in dispute could prove suspicions that high-level officials at the Justice Department knew or should have figured out the gun-walking tactics were being used.
Last week, Mr. Issa's committee, in a party-line vote, approved the contempt resolution and sent it to the full House. The six-hour committee meeting devolved into partisan bickering, with Democrats attempting parliamentary maneuvers to try to draw attention to gun-walking tactics used during ATF investigations in the George W. Bush administration.
A separate probe by the Justice Department's inspector general is nearing completion and could within weeks provide the first definitive account of what went wrong in Fast and Furious.
The confrontation is unfolding under the watch of gun-rights groups such as the National Rifle Association, which has warned lawmakers it will use their votes on the Holder contempt resolution to grade them before the November elections. Such grades are important to many lawmakers from conservative districts. A small group of Democrats is expected to vote with Republicans in support of contempt.
House Speaker John Boehner's office issued a statement Tuesday rejecting the White House's executive privilege assertions. "There are two—and only two—explanations: either the documents requested will show White House involvement in the fallout and cover-up of the operation, or the president's executive privilege claim is frivolous," Mr. Boehner said.
On Wednesday, Democrats pounced on comments from Mr. Issa appearing to undercut his earlier statements that Mr. Holder's possible involvement is one reason for pursuing more documents. Mr. Issa said, "We have no evidence nor do we currently have strong suspicion" that Mr. Holder knew about the gun-walking, though he asserted that the fight remains about documents the Justice Department won't turn over.
Maryland Rep. Elijah Cummings, the top Democrat on the House Oversight panel, appealed to Mr. Boehner to halt the contempt vote because he said the resolution had "significant legal deficiencies and factual errors."
Under federal law, if approved, the House contempt citation goes next to the U.S. attorney for the District of Columbia, Ronald Machen, who would be responsible for bringing the matter to a grand jury and beginning possible criminal prosecution of his boss, Mr. Holder.
The Justice Department's chief precedent on the issue is a 1984 opinion issued by the Office of Legal Counsel. The opinion by Ted Olson, highly regarded among conservatives, says a U.S. attorney "is not required to refer a congressional contempt citation to a grand jury or otherwise to prosecute an executive branch official who carries out the President's instruction to invoke the President's claim of executive privilege before a committee of Congress."
People familiar with the matter said that precedent is expected to help form the Justice Department response.
The last time Mr. Olson's opinion came into play was in February 2008, when the Democratic-controlled House voted to hold Josh Bolten and Harriet Miers, two senior aides to President George W. Bush, in contempt of Congress over another documents dispute. On Feb. 29, 2008, then-Attorney General Michael Mukasey wrote to then-House Speaker Nancy Pelosi that Justice wouldn't take any action against the two aides.
House Republicans are threatening to do what Democrats did in their fight with Mr. Bush by filing a civil lawsuit. That earlier dispute was eventually settled when the Obama administration took office and helped referee a settlement that included turning over some of the documents the Democratic House members sought.
Write to Evan Perez at [email protected]