Sen. Russell Feingold, D-Wis., said Congress needs to know whether some of the czars make policy but have no obligation to submit to congressional questioning.
While the Obama administration is hardly the first to name high-level advisers to handle issues like health care and climate change, Feingold said, "It's not good enough to simply say, 'Well, George Bush did it too.'"
Prior to a Senate Judiciary subcommittee hearing that featured academic experts, Feingold released a letter from White House counsel Gregory Craig that defended the officials.
Craig said some presidents have used such special advisers, or czars, to undermine Congress, but "that is simply not the case in the current administration."
Feingold also was critical of the administration for declining to send a witness to the hearing.
"The White House decided not to accept my invitation ... to explain its position on the constitutional issues we will address today," Feingold said, referring to the Senate's role in confirming top officials.
"That's unfortunate. It's also a bit ironic since one of the concerns that has been raised about these officials is that they will thwart congressional oversight of the executive branch."
Craig's letter broke down the roles of 18 officials questioned by members of Congress.
Eight are in federal agencies whose employees testify regularly before Congress. This group includes Richard Holbrooke, the Afghanistan czar and Ron Bloom, the car czar.
Four more are in the National Security Council, individuals who have no independent authority and whose sole function is to advise the president.
Another four are in the president's and vice president's offices and function as senior White House advisers on health, energy and environment, urban affairs and domestic violence. They are Lynn Rosenthal, domestic violence; Carol Browner, energy and environment; Adolfo Carrion Jr., urban affairs and Nancy-Ann DeParle, health.
Sen. Tom Coburn, R-Okla., said the administration has created doubt about its promise of transparency. However, neither Coburn nor Feingold would criticize any specific official.
It's not even clear what constitutes a czar.
"'Czar' is not an official government title of anybody; it is a vernacular of executive branch public administration," said Bradley Patterson, a hearing witness who has served on the White House staff under Republican presidents Dwight Eisenhower, Richard Nixon and Gerald Ford.
"It is a label now used loosely hereabouts, especially by the media," Patterson said.