WASHINGTON – Democratic senators who have been unable to paint Supreme Court nominee Samuel Alito as a conservative ideologue have turned to another tactic that has served them well in recent weeks: accusations of ethical impropriety. Those efforts could prove less successful against Judge Alito than similar ones against top Republicans have been, if similar questions about past Supreme Court nominees are any guide.

The ranking Democrat on the Senate Judiciary Committee, Senator Leahy, of Vermont, raised questions last week about Judge Alito’s participation in a case that involved a financial services company with which he has investments. Senator Schumer raised the same issue in a private meeting with Judge Alito yesterday. Later in the afternoon, all nine Democrats on the committee sent a letter to the lead judge on the court that heard the case, requesting information related to it.

The Democratic senators asked the chief judge on the federal Court of Appeals for the 3rd Circuit, Judge Scirica, to release internal documents that would help explain why Judge Alito did not recuse himself from a 2003 case involving three subsidiaries of the Vanguard Group, an investment firm that owns dozens of mutual funds. Judge Alito, who owns shares in 17 Vanguard funds, said in a 1990 questionnaire that he would sit out any case involving the giant financial services and investment firm.

“We regret the necessity to trouble you with this request, but under the circumstances, we hope you can understand its importance and urgency,” the senators wrote to Judge Scirica.

The letter, and the underlying ethical question it aimed to address, bore similarities to concerns that have been raised about two current justices on the Supreme Court, Justice Breyer and Justice Ginsburg. Senator Metzenbaum, a former Democratic senator from Ohio, pressed Justice Breyer during his confirmation hearing in 1994 on an investment he had at the insurance firm Lloyds of London. Justice Ginsburg has come under scrutiny for her participation in a number of cases involving companies in which her husband owns stock.

Mr. Metzenbaum argued that Judge Breyer should have recused himself from a case in which he upheld a ruling that allowed a company that had polluted 43 acres of land in New Hampshire to clean up less of the polluted site than the Environmental Protection Agency had recommended. Judge Breyer responded by saying that he was not required to recuse himself from the case since no decision would have had a substantial impact on his investment.

“Having looked at that case before and looked at it again, it seems to me that it was correct under the cannons that I could sit in that case,” Judge Breyer said at the time. “I do think that, though I understand in fact the various problems you have raised.” The Senate Judiciary Committee voted unanimously to confirm Judge Breyer.

Justice Ginsburg has been questioned about her participation in nearly two dozen cases involving companies in which her multimillionaire husband, Martin Ginsburg, has investmented. Justice Ginsburg has brushed off such questions. Asked during her 1993 confirmation hearing whether her husband’s work as a tax lawyer would be reason for recusal, she said: “As Marty gets older, he’s spending more time in the kitchen and less time with the tax business.”

A potential complication for Judge Alito is that he may have volunteered to maintain a stricter standard of recusal than ordinarily applies. In saying in the 1990 questionnaire that he would recuse himself from any case involving “The Vanguard Companies” rather than the specific mutual funds of which he owns shares, Judge Alito may have exposed himself not so much to charge of impropriety, but to the charge of not keeping his word.

Senate Democrats have signaled that they intend to pursue the issue, though Senate staffers privately concede that oversights in recusal matters are common.

A professor of judicial ethics at George Washington University Law School, Thomas Morgan, made a similar claim in a letter this week to the Republican chairman of the Judiciary Committee, Senator Specter, of Pennsylvania. Mr. Morgan said that even if Judge Alito meant to recuse himself from cases that he was not required to, the committee to which he submitted the questionnaire should not be expected to enforce the higher standard.

“Even if Judge Alito had undertaken special responsibilities involving disqualification in cases in which Vanguard appeared in any form, I do not see his failure to recuse himself in the Monga case as other than one of the inadvertent failures to disqualify that occur from time to time because of the volume of cases and press of business in the federal courts.”