WASHINGTON — How unusual is the Republican blockade of the nomination of Judge Merrick B. Garland, President Obama’s pick for the Supreme Court? After a comprehensive look at every past Supreme Court vacancy, two law professors have concluded that it is an unprecedented development.

Senate Republicans say they will not consider any nominee offered by Mr. Obama to replace Justice Antonin Scalia, who died in February. The power to appoint Justice Scalia’s successor, they say, should belong to the next president.

That categorical stance is new in the nation’s history, the professors, Robin Bradley Kar and Jason Mazzone, wrote in a study published online by The New York University Law Review. The Senate has never before transferred a president’s appointment power in comparable circumstances to an unknown successor, they said — an argument that many Democratic lawmakers have also made.

In every one of the 103 earlier Supreme Court vacancies, the professors wrote, the president was able to both nominate and appoint a replacement with the Senate’s advice and consent. This did not always happen on the first try, they wrote, but it always happened.