“Concern level” lowered. But will her constituents feel this is enough to justify voting for her again?

Senator Susan Collins of Maine plans to vote to confirm Judge Ketanji Brown Jackson to the Supreme Court, ensuring that President Biden’s nominee and the first Black woman to be put forward for the post will receive at least one Republican backer.

“I have decided to support the confirmation of Judge Jackson to be a member of the Supreme Court,” Ms. Collins said in an interview after the meeting.

The centrist senator, often a key vote on Supreme Court clashes, said that she had been reassured that Judge Jackson would not be “bending the law to meet a personal preference” and that the nominee met her personal standard for serving on the court.

“In recent years, senators on both sides of the aisle have gotten away from what I perceive to be the appropriate process for evaluating judicial nominees,” she said. “In my view, the role under the Constitution assigned to the Senate is to look at the credentials, experience and qualifications of the nominee. It is not to assess whether a nominee reflects the individual ideology of a senator or would vote exactly as an individual senator would want.”

Her decision will allow Mr. Biden and Senate Democrats to claim some degree of bipartisanship around the historic nomination, though whether other Republicans will join Ms. Collins remains unclear.

If Democrats stay united, it would also avoid the spectacle of Vice President Kamala Harris having to break a tie to seat a nominee on the Supreme Court, an unprecedented outcome that some saw as potentially damaging to the court’s standing.

Ms. Collins, who sat down with Judge Jackson for about 90 minutes before last week’s hearings, had a second, hourlong in-person meeting with the judge on Tuesday afternoon in which the two hashed out several issues that came up before the Judiciary Committee.

During the hearings, Republicans on the panel raised questions about Judge Jackson’s sentencing history on child sex abuse defendants, and tried unsuccessfully to get her to express an opinion about whether seats should be added to the Supreme Court, as some progressives have advocated. Top Republicans, including some who were regarded as potential votes for Judge Jackson, have seized on her refusal to provide an answer on expanding the court as an obstacle to her confirmation.

“I don’t understand that, because it’s not an issue that will come before her in the court, so she should as a nominee be able to talk about it,” Senator Rob Portman, Republican of Ohio, said Sunday on NBC’s “Meet the Press.” “And I’m concerned that she’s not been willing to do that.”

Republicans also seized last week on a legal brief that Judge Jackson filed on behalf of terrorism detainees at Guantánamo Bay, Cuba, which accused former President George W. Bush of having committed war crimes when his administration tortured detainees. Ms. Collins said Judge Jackson had explained during their meeting on Tuesday that she had not intended to accuse Mr. Bush personally of being a war criminal, but had used a common template for such cases.

“There can be no question that she is qualified to be a Supreme Court justice,” said Ms. Collins, citing Judge Jackson’s “breadth of experience as a law clerk, attorney in private practice, federal public defender, member of the U.S. Sentencing Commission and district court judge for more than eight years.”

Judge Jackson also met on Tuesday with Senator Mitt Romney, Republican of Utah, who has been seen as a possible vote in her favor, though he opposed her nomination to the appeals court. Senator Lisa Murkowski of Alaska, who backed Judge Jackson’s confirmation to the appeals court, is also considered a potential Republican supporter, and has yet to make her position known, saying only that her prior support is no indicator of how she will vote this time.

The White House and Senate Democratic leaders have said that confirming Judge Jackson without Republican support would have sent an unfortunate message about what they see as an overdue step to diversify of the court.

“I sincerely hope that we have Republican support for her nomination,” Senator Richard J. Durbin, Democrat of Illinois, said on Tuesday, noting that he had been “quietly” reaching out to Republican senators. “There are those within the Republican Party, I know from speaking to them, who understand the history, the significance of this nomination and want to make sure that Mr. Lincoln’s party, the Grand Old Party, is on board.”

Ms. Collins said the nomination process had become overly partisan.

“I don’t expect that any of the justices I am going to agree with on every decision — that’s impossible,” she said. “But I do want them to be able to be devoid of prejudgement, partisanship, preference and to be impartial and rule consistent with legal precedent, the language of the law and the Constitution.”