(Susan Walsh/AP)

Rep. Darrell Issa (R-Calif.) released his sixth and final report on the Internal Revenue Service targeting scandal on Tuesday, charging the tax agency with allowing a “culture of bias” against conservative groups.

The 226-page report from the chairman of the House Oversight and Government Reform Committee is Issa’s last stand on a controversy he elevated to a cause celebre over 18 months, and now passes to his successor in the next Congress. The report is a summation, repeating many previously published claims that political bias influenced the IRS’s decision to hold up screening of nonprofit advocacy groups for deeper review based on their names and policy positions.

Most of the affected groups were conservative, although a small number were left-leaning. The practice, blamed on a few employees and on management failures, has since stopped, according to the agency and its inspector general.

House Republicans have alleged that the Obama administration used the IRS and other enforcement agencies to silence conservative critics during the 2010 and 2012 election cycles, when the targeting occurred.

The report failed to show the White House coordinated with IRS officials to target conservative groups, a charge several Republican lawmakers made when the controversy first broke. In June 2013, House Appropriations Committee Chairman Hal Rogers (R-Ky.) told Fox News he hoped to discover “the enemies list out of the White House that IRS was engaged in shutting down or trying to shut down the conservative political viewpoint across the country — an enemies list that rivals that of another president some time ago.”

Issa, who suggested then-White House press secretary Jay Carney was a “paid liar” for suggesting a small number of IRS officials pursued this strategy on their own, made it clear the administration kept the committee from reaching a final answer on the question.  “The White House’s obstruction not only violated the president’s promise of cooperation, but it affected the committee’s fact-finding obligations.”

Incoming committee chairman Rep. Jason Chaffetz (R-Utah) and his staff will almost certainly continue the task of sifting through 30,000 e-mails from former tax exempt chief Lois Lerner that were lost in a computer crash. The IRS inspector general’s office says it has found the e-mails on backup tapes.

Lerner, who became a lightning rod for Republicans when she asserted her Fifth Amendment right not to testify before Congress, maintains her innocence. She retired in 2013.

Tuesday’s report says IRS employees in the office that reviews applications for tax-exempt status let their political beliefs influence their jobs, allowing their liberal bias to influence decisions on whether groups should obtain tax-exempt status.

Democrats have defended the IRS against Republican attacks, although they have condemned the practice of added scrutiny given to conservative groups.

In a statement, Rep. Elijah Cummings (D-Md.), the committee’s top Democrat, said: “It is revealing that the Republicans — yet again — are leaking cherry-picked excerpts of documents to support their preconceived political narrative without allowing committee members to even see their conclusions or vote on them first.”