"At a time when the president is seizing unprecedented power, the Supreme Court needs to act as a check and to provide balance," Leahy, the ranking Democrat on the Judiciary Committee, said in a speech at Georgetown University's law school. "Based on the hearing and his record, I have no confidence that Judge Alito would provide that check and balance."
During the final day of Alito's confirmation hearing, Leahy expressed doubt in Alito and made an almost identical statement, CBS News correspondent Thalia Assuras reported.
"How can we be sure he's going to be a check and balance?" Leahy said.
It is no surprise that the Vermont senator is voting against Alito. He was one of the nominee's harshest interrogators at Alito's confirmation hearings last week.
The Judiciary Committee is set to vote on Alito's nomination next week with the full Senate acting on the nomination a few days later.
Leahy also has been a critic of Bush's actions in the war on terror, including warrantless wiretapping on American citizens.
"In an extraordinary era of governmental intrusions into the lives of ordinary Americans, when it has been left to the Supreme Court to restore balance, it is difficult to have any confidence that this nominee will serve as an effective check on his patron or on the government which he has spent his entire legal practice representing," Leahy said.
He became the latest Democrat to announce a position on Alito, a former U.S. attorney and lawyer for the Reagan administration. Democrats Edward Kennedy of Massachusetts, Barbara Mikulski of Maryland and Max Baucus of Montana announced their opposition to Alito on Wednesday.
Leahy and Baucus both voted for Chief Justice John Roberts' confirmation last year, while the others opposed him.
Sen. Ben Nelson of Nebraska is the only Democrat to announce that he will vote for Alito's confirmation next week. He also voted for Roberts' confirmation.
The Judiciary Committee is expected to vote Alito's nomination out of committee on Wednesday in a party-line vote. The full Senate is expected to debate and vote on the nomination that same week, with its Republican majority confirming the federal appeals judge.
Alito is continuing to visit Democratic and Republican senators, spending time with Sen. Tom Carper, D-Del., and new Sen. Robert Menendez, D-N.J., Thursday morning. Carper has not taken a position on Alito's nomination.