Roberts did not mention his work on the case in responding to a Senate Judiciary Committee questionnaire that asked for examples of his pro bono work. Roberts' involvement was first reported Wednesday by the Los Angeles Times.
Jean Dubofsky, the lead lawyer for gay rights activists challenging the Colorado initiative, told The Times that Roberts gave her "absolutely crucial" advice on how to argue the case before the Supreme Court.
White House spokeswoman Dana Perino said Roberts spent less than 10 hours on the case, compared with more than 200 hours he spent on two pro bono cases on which he was the lead counsel.
"There is clearly a difference" between his assistance in the Colorado case and his other pro bono work, Perino said.
Still, the revelation is awkward for both sides in the political battle over Roberts, now a judge on the U.S. Court of Appeals for the District of Columbia Circuit. President Bush nominated him in late July to succeed Justice Sandra Day O'Connor, who is retiring.
Some conservative activists have expressed concerns that Roberts may become an "unreliable" justice like David H. Souter or Anthony M. Kennedy, who were appointed by Republican presidents but who have not consistently supported conservative positions on the bench.
Roberts' role in the Colorado case could fuel such worries.
But Grover Norquist, president of Americans for Tax Reform and a prominent GOP activist, said withdrawing support of Roberts over his participation in the case would be wrongheaded.
"The whole case that conservatives have been trying to make is that your personal feelings ought not to count" when you rule from the bench, Norquist said.
Some liberal activists consider Roberts to be more of a conservative ideologue than he acknowledges, citing in particular his work in the Reagan administration to limit school busing and other programs that sought to promote civil rights.
Roberts' work on behalf of the gay activists would be inconsistent with that view of him.
"I'm interested in seeing his involvement in this [Colorado] case, but it doesn't diminish our overall concern," said Nan Aron, president of the liberal Alliance for Justice. "A picture is just beginning to emerge of his views, but it's too early to make an overall assessment of his judicial philosophy."
Norman J. Ornstein, a fellow at the American Enterprise Institute who is an expert on relations between the White House and Congress, said news of the Colorado case "reinforces the notion that the guy is a lawyer's lawyer."
