At Wednesday's hearing, they accused him of evasive answers and challenged his stand on abortion and past membership in a conservative Ivy League alumni group.
Shortly before the hearing recessed for the day, Lindsey Graham, a Republican senator from South Carolina, chided Democrats for their tactics and Alito's wife, Martha-Ann, tearfully left the hearing room, returning about an hour later to a seat behind her husband.
"I'm not any kind of a bigot," Alito said after he had been pressed repeatedly about his membership 20 years ago of the alumni group that opposed efforts to admit more women and minorities to Princeton University.
"I believe you," Graham said. "I am sorry that you've had to go through this. I am sorry that your family has had to sit here and listen to this."
George Bush, the US president, has nominated Alito, 55, a federal appeals judge for 15 years, to replace retiring Justice Sandra Day O'Connor, who has often been the swing vote on abortion and other social issues on the nine-member court.
While Alito appeared headed for confirmation by the full Republican-led Senate later this month, several Democrats made clear that after a relatively gentle start to proceedings, they were waging an election-year fight.
Richard Durbin, an Illinois Democrat, kicked off the third day of the Senate Judiciary Committee hearing, saying he was troubled that Alito had not disavowed a 1985 memo in which he wrote that "the Constitution does not protect a right to an abortion".
"I'm concerned that many people will leave this hearing with a question as to whether or not you could be the deciding vote that would eliminate the legality of abortion," Durbin said.
Kennedy pressed Alito for his
views on affirmative action
Alito, who wrote the memo as a Reagan administration attorney 20 years ago, has not said how he would rule if abortion came before him on the high court.
But the nominee reaffirmed his vow to keep an open mind and respect legal precedent and noted that the landmark decision in 1973 that legalised abortion had been upheld repeatedly.
Charles Schumer, a New York Democrat, said: "Judge Alito has responded, but he has not answered."
Democrats continued to raise the abortion issue with Alito, having got no clear statement on whether he would vote to overturn the 1973 ruling.
After effectively parrying the question the preceding day, Alito repeated earlier responses, increasing frustration for Democrats who fear that he will push the high court to the right if confirmed.
Patrick Leahy of Vermont, the committee's senior Democrat, said Democrats were also troubled by what they saw as inconsistencies in many of Alito's answers, from abortion rights to presidential powers to membership of the Princeton alumni group.
Edward Kennedy, a Massachusetts Democrat, got into a dispute with Arlen Specter, a Pennsylvania Republican, over committee access to records of the disbanded group called Concerned Alumni of Princeton, or CAP.
By the end of the day, a bipartisan review of them had begun.
Bush picked Samuel Alito (R) to
replace Sandra Day O'Connor
Alito listed membership of the group in a 1985 application for a job in the Reagan administration. He told the panel that he had no recollection of any involvement with the group.
Kennedy quoted a 1983 essay, In Defence of Elitism, from the alumni group's magazine that read, in part: "People nowadays just don't seem to know their place. Everywhere one turns, blacks and Hispanics are demanding jobs simply because they're black and Hispanic."
Alito, a member since 199O of the 3rd US Circuit Court of Appeals, denounced the essay as offensive and said he did not know the group had promoted such positions.
Bill Frist, a Tennessee Republican and the Senate majority leader, said: "As a Princeton alumnus, I had concerns about CAP, but I have no concerns about Judge Alito's credibility, integrity and his commitment to protecting the equal rights of all Americans."