WASHINGTON — Senator Collins of Maine said Wednesday she will vote to confirm Judge Ketanji Brown Jackson, giving Democrats at least one Republican vote and all but assuring that Judge Jackson will become the first Black woman on the Supreme Court.

Ms. Collins said in a statement that she met with Judge Jackson a second time after four days of hearings last week and decided that “she possesses the experience, qualifications, and integrity to serve as an Associate Justice on the Supreme Court.”

“I will, therefore, vote to confirm her to this position,” Ms. Collins said.

Her support gives Democrats at least a one-vote cushion in the 50-50 Senate and likely saves them from having to use Vice President Harris’ tie-breaking vote to confirm President Biden’s pick. It is expected that all 50 Democrats will support Jackson, though one notable moderate Democrat, the senator from Arizona, Kyrsten Sinema, hasn’t yet said how she will vote.

Judge Jackson, who would replace retiring Associate Justice Stephen Breyer, would be the third Black justice, after Justices Thurgood Marshall and Clarence Thomas, and the sixth woman. She would also be the first former public defender on the court.

Ms. Collins was the most likely Republican to support Judge Jackson, and she has a history of voting for Supreme Court nominees picked by presidents of both parties. The only nominee she’s voted against since her election in the mid-1990s is Associate Justice Amy Coney Barrett in 2020.

She said in the statement that she doesn’t expect that she will always agree with Judge Jackson’s decisions.

“That alone, however, is not disqualifying,” Ms. Collins said. “Indeed, that statement applies to all six Justices, nominated by both Republican and Democratic Presidents, whom I have voted to confirm.”

It is unclear if any other GOP senators will vote for Judge Jackson. Senator McConnell said last week he will not support her, citing concerns about her sentencing record and her support from liberal advocacy groups.