House Democrats say Rep. Darrell Issa (R-Calif.) has refused to allow one of their invited witnesses to testify at a hearing on Thursday.

Democrats on the House Oversight and Government Reform Committee invited Fred Wertheimer, president of campaign finance watchdog Democracy 21, to testify at a joint hearing hosted by the Oversight panel and the House Small Business Committee. The two committees are looking into President Obama’s draft executive order that would force government contractors to disclose their political contributions.

Issa, Oversight’s chairman, has denied Wertheimer a seat at the witness table, according to Rep. Elijah Cummings (D-Md.), the committee’s ranking member. That has left only one minority-invited witness: Lawrie Hollingsworth, who is testifying on behalf of the U.S. Women’s Chamber of Commerce and was invited by Democratic members of the Small Business panel.

A total of seven witnesses will appear at the hearing.

Wertheimer was expected to testify on behalf of a coalition of public interest groups that support the draft order.

“It is deeply troubling that Chairman Issa refused to allow testimony from this coalition of independent experts,” said Cummings in a statement. “Denying their testimony is a disservice to members of Congress and the public, and it tarnishes the integrity of the committee. Our committee should be a model for others to follow. Especially in a debate about openness and transparency, we should be open and transparent ourselves."

A spokesman for Issa said Republicans accepted the first request for a minority witness it received, which was for Hollingsworth, at the request of the Small Business panel's ranking member, Rep. Nydia Velazquez (D-N.Y.).

Spokesman Frederick Hill said the Obama administration's viewpoint should be provided by another witness, Daniel Gordon, the Office of Management and Budget's administrator for federal procurement policy.

"Ranking Member Cummings has only himself to look at for his failure to coordinate his minority witness request with his Democratic counterparts on the Small Business Committee. Whatever this committee does, whoever it invites, Ranking Member Cummings always looks for a way to obstruct and whine," said Hill.

The decision not to allow Wertheimer to testify comes after Cummings wrote to Issa on Tuesday asking the committee chairman to let a witness from Democracy 21 testify.

“The witnesses you have invited, however, appear to represent only one side of the debate. I am concerned that such an unbalanced approach would not provide members an accurate or complete understanding of this issue,” Cummings wrote in his letter.

Wertheimer supports the draft order, alongside several other public interest groups. They argue the order would increase transparency and help show who was funding outside groups that didn’t have to disclose their donors. Such groups were prevalent in the last campaign season.

The draft order has attracted intense opposition from Republicans and several business groups, such as the U.S. Chamber of Commerce and the Business Roundtable. They charge that the order would inject politics into the federal contracting process, allowing the Obama administration to target GOP supporters.

The draft order comes after the White House failed last Congress to pass the Disclose Act — legislation that would have had outside groups disclose their donors — when it stalled in the Senate. That was in response to the Supreme Court’s Citizens United decision in January 2010 that allowed corporations and unions to spend unlimited funds on electioneering activities and set off a wave of advertising before the midterm elections.

This post was updated at 10:43 a.m.