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Legal experts testifying before the Senate panel said Congress needs to be careful not to overreach in reacting. They said options open to lawmakers include writing new laws to restrict advisers’ authority or writing the positions into law as needing Senate confirmation. A White House aide, speaking on the condition of anonymity, said they are trying to work to accommodate “all reasonable congressional requests for information” and said some White House advisers have given informal briefings to members of Congress in lieu of testimony.

The aide also said some czars are outside the White House itself and they can be called to testify. The aide said five of them have already done so.

Sen. Claire McCaskill, Missouri Democrat, blasted Republicans for raising the issue and said she took offense at their comparisons between Mr. Obama and President Nixon.

She said Congress should instead be looking at the legality of presidential signing statements to shape how laws are implemented - a tool whose use expanded substantially under President George W. Bush.

Last month, Miss Collins offered an amendment to compel administration officials to testify, but it was ruled not germane to the bill being debate.

Democrats said it went too far because it would have covered all executive branch employees, including the national security adviser and the chief of staff, who have always been recognized as out of bounds.

Miss Collins said the issue shouldn’t be so intractable and that Congress and the White House should be able to agree on a list of people who should be able to testify.

For his part, Mr. Lieberman said he’s still looking for a good solution.

“We both share a desire to do something about this to help Congress uphold our responsibility for oversight, but we understand the balance here as reflected in the Constitution,” he said.