Supreme Court nominee Samuel Alito said Thursday he was "unduly restrictive" in promising in 1990 to avoid appeals cases involving two investment firms and said he has not made any rulings in which he had a "legal or ethical obligation" to step aside.

In a letter to Sen. Arlen Specter, chairman of the Judiciary Committee, Alito said a 1990 questionnaire he filled out for the panel covered his plans for "initial service" as a judge on the 3rd U.S. Circuit Court of Appeals.

"I respectfully submit that it was not inconsistent with my questionnaire response for me to participate in two isolated cases seven and 13 years later, respectively," he wrote.

Alito issued the letter one day after all eight Democrats on the committee called for voluminous records involving a 2002 case in which Vanguard was a defendant. They noted that Alito had promised at the time of his confirmation to the appeals court to avoid cases involving Vanguard, Smith Barney, First Federal Savings & Loan of Rochester, N.Y., and his sister's law firm.

Democrats addressed their letter to the chief judge of the 3rd circuit, and did not accuse Alito of bending his ruling to favor Vanguard. Instead, they raised possible conflict of interest concerns, and said he had violated the promise he made to the committee 15 years ago.

Even so, the Democratic challenge prompted Specter, R-Pa., to write Alito suggesting a quick response. "I think it is important that the issues be addressed promptly since a number of senators have expressed concerns," Specter advised.

Alito has been nominated to succeed Justice Sandra Day O'Connor, whose judicial rulings have made her the swing vote on the Supreme Court on issues such as abortion and affirmative action. The 55-year-old New Jersey native has won strong support from Senate Republicans, and elicited praise from several Democratic senators with whom he has met privately.

Confirmation hearings are scheduled for Jan. 9, and Wednesday's letter from Democrats on the panel marked their first organized challenge to his nomination.

Specter warned Alito in his letter that waiting to answer the conflict of interest questions would only allow "your adversaries to speculate on this issue to the detriment of your nomination."

The senator said he thinks "there has been no impropriety." But he added that "we have seen issues which may be minor, unmeritorious and even nonexistent, proliferate into major controversies by those who are opposed for other reasons."

Alito responded within a few hours, suggesting he knew the letter was coming, or even that the White House had sought an invitation to respond to the issue. In the letter, Alito said he only invests with Vanguard and Smith Barney and doesn't hold any interest in the companies.

When he listed the companies in the 1990 questionnaire, "my intention was to state that I would never knowingly hear a case where a conflict of interest existed. ... As my service continued, I realized that I had been unduly restrictive," Alito said.

In the 2002 case, Shantee Maharaj, who had lost a suit against Vanguard, sought a rehearing after learning Alito held investments with the mutual fund company. She sought to have the ruling erased and Alito disqualified from further proceedings.

Alito said Thursday he did not believe he was required to disqualify himself on the basis of ownership of shares in a mutual fund but "voluntarily recused myself once my participation was called into question."

The White House and senators have said that there was a computer glitch that allowed the recusal problem to slip through undetected in the first place, but Alito only mentioned an "oversight" in the letter to Specter.

Alito's letter also said he decided a case involving Smith Barney, but added it did not fall under judicial conflict-of-interest guidelines. "Smith Barney is my brokerage firm and I hold no interest in the firm itself," the judge said in the letter.