The vow by Senate Republicans to block whomever President Barack Obama nominates to fill the sudden vacancy on the Supreme Court presages a prolonged election year struggle, but the clash is less a new front against the White House than an escalation of a battle that had begun at the appeals court level before Justice Antonin Scalia died.

Since Republicans took control of the Senate in January 2015, the process that would enable Obama to fill vacancies on the 12 regional federal courts of appeal has essentially been halted. Obama has managed only one appointment because Republican senators have refused to sign off ahead of time on nominees for judgeships in their states — a traditional step before a president makes a nomination.

In the weeks before Scalia’s death, influential conservative groups and commentators called on Senate Republicans to ensure that Obama appointed no more appeals court judges.

Among those commentators was Ed Whelan, a former clerk to Scalia and a prominent commentator. He said in an interview Monday that conservatives cannot compromise over any appointments to the upper ranks of the judiciary — including the appeals courts, which get the last word on matters the Supreme Court does not review and often serve as a breeding ground for future justices.

“This fight has been fought by both sides for decades,” Whelan said. “Conservatives believe with good reason that liberal judges will twist the Constitution and statutes to reach whatever result they want.”

History is no longer a guide

Just as there is no precedent for leaving a Supreme Court seat open because it is an election year, as Sen. Mitch McConnell, the majority leader, vowed to do hours after Scalia’s death was announced, there is also none for virtually shutting down the appointment of new appeals court judges so early. Each of Obama’s predecessors since Ronald Reagan also faced a Senate controlled by the opposing party, yet they appointed between 10 and 18 appellate judges in their last two years in office.

But history is no longer a guide in a polarized Washington, where partisan warfare over judicial nominations has been escalating for more than a generation.

“The current picture represents the continuing decline in the Senate’s ability to perform what was once a routine function — consenting to the appointment of qualified judicial nominees,” said Russell Wheeler, a Brookings Institution scholar who studies confirmations.

Payback for 2013 rule changes

Republicans have framed their resistance as payback for the decision by Democrats in 2013 to change Senate rules and eliminate the ability of lawmakers in the minority to block confirmation votes with a filibuster. Republicans had used the tactic with unprecedented frequency while in the minority to slow or block Obama’s nominees as part of a tit-for-tat dating to 1987, when Senate Democrats rejected Reagan’s nomination of Robert H. Bork to the Supreme Court.

Last June, a conservative radio host referred to the rule change in an interview with McConnell, and asked how the new Republican majority was handling judges.

McConnell replied that “so far, the only judges we’ve confirmed have been federal district judges that have been signed off on by Republican senators,” adding that it was “highly likely” the pattern would remain for the rest of the 114th Congress.

At the time, there were eight vacancies on the appeals courts, but Obama had submitted the name of only one nominee: Luis Felipe Restrepo, a District Court judge in Philadelphia. He had Republican backing, and the Senate confirmed him last month.

Obama submitted no other names, according to administration officials, because the vacancies were in states that had at least one Republican senator, and those senators had refused to preapprove any nominee.

Breaking with tradition

Traditionally, preapproval is part of the nomination process. The Senate Judiciary Committee generally does not schedule a hearing for a nominee without the consent of both senators from the state in which the seat is based, regardless of party.

Starting last month, Obama quietly broke with that tradition. He has now submitted nominees to fill four of those longstanding vacancies, even though none had pre-approval from Republican senators. In an interview last week, Neil Eggleston, Obama’s White House counsel, said the president decided to move forward because he hoped Republican senators would permit at least some to go through.

“The calendar was running out, and it was time to get moving,” Eggleston said. “At some point the process just has to get started.”

Last week, for example, Obama nominated Abdul Kallon, a District Court judge in Alabama, for an Alabama-based appeals court seat vacant since October 2013. Alabama’s senators, Richard C. Shelby and Jeff Sessions, both Republicans, had supported making Kallon a district court judge in 2009, but they did not agree to elevating him.

In a speech to a Republican club in Alabama in January, Shelby said that he and Sessions were working to prevent Obama from filling any more of Alabama’s openings on the bench. When Obama announced the nomination last week, the two senators said they would oppose Kallon.

Sen. Grassley backs up McConnell

After McConnell’s call to leave Scalia’s seat open until the next administration, Sen. Charles E. Grassley, the Iowa Republican who is chairman of the Judiciary Committee, voiced his agreement. But even before Scalia’s death, he had made clear his opposition to moving any appeals court nominees  in states where both senators did not sign off.

In response last week to a request for an interview on the subject of appeals court judges, Grassley pointed to a January speech in which he said his committee “will continue to hold hearings this year on judicial nominees and we’ll continue to do our due diligence in evaluating those nominees.”

But Beth Levine, a spokeswoman for Grassley, added later that he “is following the longstanding precedent of the Senate, observed by both Democrats and Republicans, that absent signoff from both of the home state senators, the nominee does not go forward.”

Pendulum swung after Bush presidency

When Obama took office, the appellate bench had been pushed to the right by President George W. Bush, who made filling vacancies a higher priority at the start than Obama did. Appointees of Republican presidents controlled nine of the 12 courts, while appointees of Democrats controlled one, according to data from Wheeler of the Brookings Institution.

Today, the pendulum has swung: Eight have majorities of active judges appointed by Democrats, and four are controlled by Republican-appointed judges.

(Obama has also made several appointments — including one in July 2015 — to the Court of Appeals for the Federal Circuit, which has no home state senators and provokes little controversy because it specializes in patents and claims. And Obama has appointed 264 District Court judges, with several nominees who have Republican support and may yet get confirmed. Bush appointed 261 District Court judges and President Bill Clinton 305.)

An historical anomaly

If he makes no more appointments to the regional appeals courts, Obama would leave at least 12 vacancies to his successor, counting seats that recently came open or are expected to by the end of the year. By that measure, Obama’s appeals court record would be about the same as Bush’s and better than that of Clinton — who also had trouble with a Republican Senate and left more than two dozen open.

But by other measures, Obama is on track to be an historical anomaly. He has appointed just 48 judges to the regional appeals courts so far, while Bush and Clinton each appointed more than 60.

The gap between Obama’s numbers and his recent predecessors’ occurred in the final two years of their presidencies. Obama appears likely to appoint the fewest such judges during that period of any president since Congress created the courts of appeal in 1891, with one exception: President Grover Cleveland, who named none in the two years before he left office in 1897.

But Cleveland had no vacancies to fill.