WASHINGTON (Reuters) - Republicans accused the U.S. Internal Revenue Service on Monday of hiding emails written by a former senior official and obstructing a congressional inquiry into a controversy involving past IRS treatment of conservative groups.

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In an unusually contentious evening hearing on Capitol Hill, Democrats accused Republicans of rehashing baseless accusations for political theater in the so-called IRS Tea Party targeting affair that traces back to mid-2013.

The IRS reviews the activities of non-profits that apply for tax-exemption because U.S. law limits their political involvement. Non-profits have increasingly been used as conduits for political spending, especially by conservatives.

In May 2013, Lois Lerner, who headed an IRS unit involved in applying extra scrutiny to conservative political groups’ applications for tax-exempt status, apologized in public for what she called “inappropriate” review of conservative groups’ applications. Republicans have been investigating since then.

Although Lerner retired in September 2013, the controversy leapt back into the headlines last week when the IRS said a computer crash had caused it to lose emails written by Lerner, angering Republican investigators who want the emails for review.

Representative Darrell Issa, one of the Republicans’ most aggressive congressional investigators, chaired the hearing and criticized what he called “obstruction by the IRS.”

Issa accused IRS Commissioner John Koskinen, seated just a few feet away, of working to “cover up the fact that there were missing emails”. Issa said to Koskinen: “I’m sick and tired of your game-playing in response to congressional oversight.”

A former corporate turn-around expert confirmed as IRS chief in December in the midst of the agency’s worst crisis in years, Koskinen said: “No one has been keeping this information from Congress.”

Hundreds of thousands of documents and emails, he said, have been provided to Issa’s House of Representatives Oversight and Government Reform Committee, as well as the tax law-writing House Ways and Means Committee.

The agency said last week that Lerner’s computer crashed in mid-2011 and that some of her emails from January 2009 to April 2011 could not be recovered. The IRS expects to salvage more of Lerner’s emails by the end of June and turn them over to Congress, Koskinen said.

Democratic Representative Gerry Connolly said the hearing was political theater meant to stir the Republican voter base ahead of November’s congressional elections. He called Republicans’ conduct of the hearing “reckless and disgraceful.”

Republicans have been intent on linking the IRS scrutiny to the White House, without success.

Issa’s committee said on Monday that, as part of its inquiry, it had subpoenaed Jennifer O’Connor of the White House Counsel’s office to testify on Tuesday.

The subpoena followed a letter from White House Counsel Neil Eggleston, declining to make O’Connor available on a voluntary basis to testify, the committee said.