WASHINGTON — Senator Schumer is coming under criticism from Republican members of the Senate Judiciary Committee who say Mr. Schumer’s leadership in the investigation of the U.S. attorney firings presents a conflict of interest with his role heading the Democratic Senate Campaign Committee.

Senator Specter, a Republican of Pennsylvania, said yesterday that Mr. Schumer had crossed the line, pointing out that a senator caught up in the controversy, Peter Domenici of New Mexico, is a lawmaker Mr. Schumer is aiming to oust in 2008. Mr. Specter, the former chairman of the Judiciary Committee, said he confronted Mr. Schumer about the issue last week and that the New York senator had “taken a very political stance.”

“I think that the inquiry by the Judiciary Committee ought to have at least a modicum of objectivity,” Mr. Specter said on “Fox News Sunday.”

The committee is investigating what role the White House played in the December dismissal of eight U.S. attorneys amid accusations that the moves were politically motivated. Several lawmakers, including Mr. Schumer and Senator Clinton, have called for the attorney general, Alberto Gonzales, to resign, and the committee is preparing to issue subpoenas to White House aides, including President Bush’s top political adviser, Karl Rove. The situation has worsened for the administration as recently released documents, including e-mails, have shown greater involvement by the White House in the dismissals, contrary to what the Justice Department initially told Congress.

Critics of the probe have said Democrats are making too much of the dismissals, saying previous administrations have fired dozens of federal prosecutors upon taking office, and that, under federal law, U.S. attorneys serve at the pleasure of the president.

Mr. Domenici’s involvement stems from his acknowledgement that he called the U.S. attorney in New Mexico, David Iglesias, last year to ask about the progress of an investigation into alleged voter fraud by Democrats. Mr. Iglesias, who was one of the prosecutors fired in December, has said he felt “leaned on” and that his dismissal was due to politics, not job performance.

Mr. Specter said yesterday that the day after Mr. Domenici testified before the Senate Judiciary Committee last week, he was featured on the Web site of the Democratic Senate Campaign Committee. The New Mexico senator is up for re-election next year, and the campaign committee, of which Mr. Schumer is chairman, is charged with retaining and widening the Democratic majority in the Senate. “I don’t think he can do both things at the same time without having a conflict of interest, but that’s up for him to decide,” Mr. Specter said.

Another Republican on the committee, Senator Cornyn of Texas, echoed those concerns yesterday, saying the investigation risked becoming a “political witch hunt.”

“I think it undermines the apparent legitimacy of what is a legitimate inquiry,” Mr. Cornyn said on the ABC News program “This Week.” Asked yesterday about the suggestion of a conflict of interest, Mr. Schumer was dismissive. “This is much too serious to be about politics,” he said on the NBC News show “Meet the Press.” The Judiciary Committee, he said, was only looking into actions by the executive branch, and any inquiry involving members of Congress would be handled by the Senate Ethics Committee, on which he does not serve. “So there’s no conflict whatsoever,” he said. Appearing on ABC News, the chairman of the Judiciary Committee, Senator Leahy of Vermont, defended Mr. Schumer, saying he had asked “very legitimate questions.” But he also made clear that he was in charge of the committee and would make the final decision on subpoenas.

As of yesterday, the Democratic Senate Campaign Committee Web site was not featuring Mr. Domenici, other than to include him in a map of Republican senators who are up for re-election in 2008. There was no reference to the controversy over the firing of federal prosecutors.

Mr. Schumer, who is the thirdranking Democrat in the Senate, became the first member of the leadership to call for Mr. Gonzales’s resignation last week. Many other Democrats, as well as a few Republicans, have followed suit. Mr. Schumer yesterday said he wouldn’t be surprised if Mr. Gonzales was gone by the end of the week.

Mr. Gonzales has acknowledged making mistakes but has vowed to stay on. President Bush has also supported him, although he said last week that he was unhappy about the situation.