WASHINGTON — The bitter impasse over the nomination of Judge Merrick B. Garland is raising a once unthinkable possibility: Presidents may no longer be confident of filling a Supreme Court vacancy unless their party controls the Senate.

With incumbents more worried about primary challenges and activist groups wielding greater influence, senators from both parties have shown increasing reluctance to back otherwise qualified judicial nominees whose views and records do not align with their own or their party’s.

Now the Republican decision to deny Judge Garland any consideration is taking court politics to a hyperpolarized level, one that does not bode well for the future of an already seriously troubled confirmation process.

“The problem is the whole thing is just getting ratcheted up, and maybe now we are in a new era when the opposition party will refuse to confirm anyone,” said Russell Wheeler, an expert on the federal judiciary at the Brookings Institution.