Senate confirmation of Roberts, the first of two Supreme Court selections expected from President Bush this year, is all but assured. The vote is scheduled for Thursday.
"I am mindful that diversity is one of the strengths of the country," Bush said. He said he would not make an announcement until the Senate voted on Roberts' nomination.
Roberts would replace Chief Justice William H. Rehnquist, who died Sept. 3 at age 80. Bush's next nominee would step into the seat of the first woman on the court, who was a key swing vote in a number of high-profile cases.
Mikulski said part of her reason for opposing Roberts is her hope that by doing so, she and other Democrats could influence Bush to pick a more moderate nominee for the second vacancy.
Mikulski joined 15 other Democratic senators in declaring opposition to Roberts, though her party is split.
More than a dozen Democrats have said they will vote for Roberts, and all 55 Republicans in the Senate are expected to support him, giving the 50-year-old federal appellate judge more than enough votes to guarantee his elevation to head the nation's highest court.
Although the confirmation vote has largely been drained of drama, the Senate will spend most of this week debating Roberts' nomination.
Mikulski said Roberts is a man of integrity and a competent judge. But she said his testimony before the Senate Judiciary Committee had left her uneasy about his views.
"I am left with these persistent doubts about whether he will safeguard civil rights, the right to privacy and equal protection under the law," said Mikulski, the Senate's longest-serving female senator. "It's not his demeanor we're voting on; we're voting on what will his judicial philosophy mean to the court."
Mikulski said she was concerned that Roberts refused to elaborate on his thoughts about Roe v. Wade, the watershed 1973 abortion-rights case, or about other issues involving reproductive rights.
A broad coalition of liberal groups, including those supporting abortion rights, has lobbied hard for Democrats to vote against Roberts.
Besides Mikulski, other Democrats opposing Roberts include four potential contenders for the party's 2008 presidential nomination: Sens. Hillary Rodham Clinton of New York, Evan Bayh of Indiana, Joseph R. Biden Jr. of Delaware and John Kerry of Massachusetts. A fifth Democratic presidential possibility, Sen. Russell D. Feingold of Wisconsin, has said he will vote for Roberts.
Democratic Sen. Paul S. Sarbanes of Maryland has not said how he will vote.
Mikulski called a senator's vote on a Supreme Court nomination one of two decisions that are "irrevocable and irretrievable," the other being a vote on going to war.
Mikulski said she would oppose Roberts, even though he should be confirmed easily, because she feels it is important for her concerns to be heard.
"I think you have to declare your own views and analysis," she said.