The Maine senator Susan Collins will vote to confirm Ketanji Brown Jackson to the US supreme court, giving Joe Biden’s nominee a rare Republican vote as she proceeds towards becoming the first Black woman ever to sit on the nine-judge panel.

“I have decided to support the confirmation of Judge Jackson to be a member of the supreme court,” Collins, a Republican moderate, told the New York Times after meeting the nominee a second time.

“There can be no question that [Jackson] is qualified to be a supreme court justice.”

The confirmation was not in doubt. Democrats need only their own 50 votes in the evenly divided Senate to put Jackson on the court, given the casting vote of the vice-president, Kamala Harris. Joe Manchin, a centrist Democrat from West Virginia, had already confirmed his support.

But the vice-president’s tie-breaking vote has never been required to confirm a supreme court justice, making Collins’ vote at least symbolically important.

Collins’ support also comes at a time of bitter partisan divide that was underscored by hostile and politically loaded questioning of Jackson led by white, hard-line Republican men, prominently including Tom Cotton, Ted Cruz and Josh Hawley.

Ahead of a confirmation vote next week, other Republican moderates could follow Collins and announce support for Jackson. In particular, Mitt Romney of Utah, the 2012 Republican nominee for president, has said he has not yet decided.

Jackson will replace Stephen Breyer when he retires this summer. As Breyer is a member of the outmatched liberal group on the court, his replacement will not alter the 6-3 conservative majority.

Republicans including the Senate minority leader, Mitch McConnell of Kentucky, have complained that Jackson would not take a position on calls from progressives to expand the court in order to redress its ideological balance, during confirmation hearings last week.

Speaking to the Times, Collins said: “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.

“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.”

Supreme court confirmations have become highly political and increasingly rancorous.

In 2016, as Senate majority leader, McConnell refused even a hearing for Barack Obama’s final nominee, Merrick Garland, thereby holding a seat open in the hope it would be filled by a Republican president.

Collins duly voted to confirm Donald Trump’s first nominee, Neil Gorsuch, the beneficiary of such hardball tactics.

Collins also voted to confirm Brett Kavanaugh, Trump’s second nominee, who vehemently denied accusations of sexual assault.

But Collins voted not to confirm Amy Coney Barrett, a hardline conservative installed shortly before the 2020 presidential election in place of Ruth Bader Ginsburg, a liberal lion, in stark contravention of supposed principles about confirmations close to elections laid out by McConnell four years before.

During the Democratic primary in 2020, Joe Biden promised to put a Black woman on the supreme court. Some Republicans voiced opposition to such a promise, disregarding precedent including Trump’s vow to name a woman to replace Ginsburg.

In 2021, Collins was one of three Republicans who voted to confirm Jackson to a federal appeals court. The other two were Lindsey Graham of South Carolina – a hostile questioner in Jackson’s supreme court hearings – and Lisa Murkowski of Alaska. Murkowski has not said which way she will vote this time.

Explaining her vote to the Times, Collins cited Jackson’s “breadth of experience as a law clerk, attorney in private practice, federal public defender, member of the US Sentencing Commission and district court judge for more than eight years”.

In a tweet, the White House chief of staff, Ron Klain, said: “Grateful to Senator Collins for giving fair, thoughtful consideration to Judge Jackson – and all of the president’s judicial nominations.”