WASHINGTON — Her confirmation assured, Elena Kagan is on the brink of becoming the fourth woman ever to serve as a Supreme Court justice.

The Senate is set Thursday to confirm President Barack Obama’s nominee, whose addition to the court will mark the first time three female justices have served concurrently. Nearly all Democrats, the Senate’s two independents and a handful of Republicans are backing her.

The vote is to be one of the Senate’s last actions before its members depart for a monthlong vacation.

Republicans have harshly criticized Kagan, 50, as a political activist who would be unable to put aside her liberal views and render impartial decisions. Democrats defend the former Harvard Law School dean as a highly qualified legal scholar who could help bring consensus to the polarized court.

She’s not expected to alter the ideological balance there as she succeeds retired Justice John Paul Stevens, who is regarded as a leader of the court’s liberal wing.

Kagan’s nomination to a lifetime seat on the nation’s highest court has garnered relatively little notice this summer, with the public and elected officials preoccupied by bad economic news and the Gulf oil spill, and many lawmakers nervously eyeing the November midterm congressional elections.

But senators have used the debate to press dueling visions of the Supreme Court. Democrats say Kagan would be a mainstream, moderate counterweight to a conservative majority they say has defied Congress and ignored the Constitution in its rulings on issues such as workplace rights and campaign finance.

With her confirmation, Sen. Chuck Schumer, D-N.Y., said near the end of a three-day debate, “average Americans will be a step closer to once again having their voices heard in the highest court in the land.”

Republicans argue that Obama’s choice of Kagan reflects Democratic attempts to pack the courts with liberals who will mold the law to their agendas.

Kagan “is truly a person of the political left — now they call themselves progressives — one who has a history of working to advance the values of the left wing of the Democratic Party, and whose philosophy of judging allows a judge to utilize the power of their office to advance their vision for what America should be,” Sen. Jeff Sessions of Alabama, the top Republican on the Judiciary Committee, told reporters in a conference call.

GOP senators have painted Kagan as antigun and pro-abortion. And they have denounced her decision as dean to bar military recruiters from the Harvard Law School career services office because of the prohibition against openly gay soldiers. Kagan, who called the “don’t ask, don’t tell” policy abhorrent, has said she was merely complying with Harvard’s university’s nondiscrimination policy.

When sworn in, Kagan would join two other women on the court, Ruth Bader Ginsburg and Sonia Sotomayor, who was Obama’s first nominee. Sandra Day O’Connor was the first woman appointed to the court. She served from September 1981 to January 2006.