While Democrats failed last week to upend the Senate filibuster to pass new voting rights laws, they do not have to change any rules to thwart a Republican filibuster against a Supreme Court nominee — those changes have already been made.

Beginning in 2013, feuding Democrats and Republicans enacted changes that in effect shield a nomination to the high court from a filibuster, meaning Democrats will not have to muster the 60-vote supermajority typically needed to break one and move to a final vote.

The first change came in 2013, when Democrats, stymied by Republican filibusters against President Barack Obama’s judicial nominees, unilaterally changed the rules to allow most executive branch nominations to skirt an attempted filibuster with a simple majority of 51 votes. Supreme Court nominees were not included.

After Donald J. Trump’s inauguration in 2017, Republicans moved quickly to expand the filibuster exemption to cover nominees to the Supreme Court, clearing the way for Mr. Trump to fill three vacancies and leaving Democrats with no recourse to stop him.

The filibuster change that Senators debated last week was for legislation, which is considered on a separate track from nominations. Two Democrats, Senators Joe Manchin III of West Virginia and Kyrsten Sinema of Arizona, joined all 50 Republicans in opposing that revision, halting President Biden’s voting rights bill, which lacks enough votes to overcome a G.O.P. blockade.

When Republicans blocked Mr. Obama’s nomination of Merrick B. Garland to the Supreme Court in 2016, they did not need to use the filibuster, since they had a majority in the Senate and simply refused to take up Mr. Garland’s appointment.

Though Democrats do not need to worry about Republicans using a filibuster against the forthcoming nominee, winning confirmation is no slam dunk. With the Senate split 50-50, Vice President Kamala Harris will be needed to break a tie vote, meaning Democrats will have to either hold all 50 of their members together or win backing from Republicans. In addition, the illness or even death of a single Democratic senator could deprive them of their majority and greatly complicate confirmation proceedings.

If Republicans take back control of the Senate in this year’s midterm elections, it is conceivable that they would block any nominations by Mr. Biden to the high court and try to await the outcome of the 2024 elections.