Similarly, David Camp, Republican of Michigan and chairman of the House Ways and Means Committee, questioned the process the I.R.S had used to challenge the five donors and whether the Obama administration, which has been critical of the increasing power of advocacy groups, was involved.

On Thursday, Mr. Camp issued a statement saying that while he welcomed Mr. Miller’s decision, he remained concerned that the I.R.S. had not ruled out future gift taxes on or audits of such donations. “I remain troubled that the I.R.S. has failed to explain what prompted these audits in the first place,” he said.

The I.R.S. has said that the White House had nothing to do with its audits, and its original statement said they were begun by lower-level employees and that administration officials were not notified. Frank Keith, an I.R.S. spokesman, said Thursday that the estate and gift tax unit had in fact suspended those audits in March, although the agency’s initial confirmation of the audits did not mention any such action.

“All decisions to open, suspend and close the audits were made by career civil servants and were not the result of any outside influence,” Mr. Keith said.

Mr. Miller’s memo today indicated that no new inquiries would be made of donors to such groups until these issues had been studied and a broader policy developed, on a timetable he left open-ended. Greg Colvin, a lawyer for one of the five donors under audit by the I.R.S., called the memo issued Thursday “at least a temporary victory.”

“They might sometime in the future decide to apply the tax,” Mr. Colvin added. “But in my experience, they can go for a very, very long time if they’ve got a tough issue where there is difficult public policy, contradictions, constitutional questions, debate over what Congress intended. They could keep this on ice for decades.”

Mr. Colvin and other lawyers said the I.R.S. turnaround might lead to a spate of refund requests from donors who made contributions to advocacy groups and paid gift taxes. “It makes sense to file a protective claim to avoid losing out by virtue of the statute of limitations expiring,” said Marcus Owens, a lawyer who formerly headed the I.R.S. division that oversees nonprofit groups.

Mr. Keith, the I.R.S. spokesman, said taxpayers had the right to file for a refund of any tax they had paid. “However, I’m not saying how the I.R.S. would treat such a claim, whether it would be honored or denied, and the field directive” — Mr. Miller’s memo — “doesn’t speak to that, either,” he said.