Mr. Schiliro said the group was open to working with Republicans if their interests align, but that has not yet occurred.

Among the resources the group has made available on its website, Co-Equal.org, is a guide to precedents showing how Congress has in the past pried documents from the White House and won the testimony of influential administration officials, including chiefs of staff, national security advisers and White House counsels.

“It is so that people have a factual resource to look at what has been done in the past so that when somebody makes these outrageous claims that their people should not have to testify you can say, ‘No, no, it is normal for people to have to testify before Congress,’” said Karen Lightfoot, a longtime congressional communications and policy expert who is part of the group.

Members of the group have also been providing training to lawmakers and staff members on that most essential of Capitol Hill skills: how to elicit meaningful information from well-coached hearing witnesses in five-minute rounds of questioning.

Mr. Schiliro noted that outside witnesses under scrutiny from Congress have most likely undergone hours of preparation by experienced lawyers and public relations experts — sometimes in faux hearing rooms to provide verisimilitude — while members of Congress might first be seeing their own questions prepared by staff as they take their seats at the hearing.

Another major project of the group came in the tax realm when it helped develop an analysis that showed the top 1 percent of taxpayers receiving an outsize windfall from Republican tax cuts since 2001 compared with the rest of the country — an analysis pivotal in shaping the Democratic response to the tax cuts.

“Their expertise is highly respected,” said Jonathan Davidson, the chief of staff to Senator Michael Bennet, Democrat of Colorado, who worked with Co-Equal on tax policy. “They are a small organization that is having a big impact.”