WASHINGTON (AP) — The Democrats' fragile hold on the Senate majority became even more tenuous Wednesday with the sudden illness of New Mexico Sen. Ben Ray Luján, which sent shock waves through the party and threatens President Joe Biden's Supreme Court pick and already lagging legislative agenda.

The senator's office announced that the 49-year-old remained hospitalized after suffering a stroke and is expected to make a full recovery. But Senate colleagues were blindsided by the news — even top-ranking leaders were reportedly unaware that Luján fell ill last Thursday, a stunning oversight. Without his presence, the party no longer has full day-to-day control of what has been an evenly split Senate, throwing Biden's potential Supreme Court nomination and even routine business into doubt in the face of Republican objections.

Senate Majority Leader Chuck Schumer spoke of the "awful, frightening" situation, but remained hopeful that Luján would be "back to his old self" before too long.

"The U.S. Senate will continue to move forward in carrying out its business," Schumer said Wednesday. "All of us are rooting for him every step of the way — between now and the day he makes his return to the Senate."

The uncertainty shows just how precarious the Democrats' hold on power in Washington really is and the limits of Biden's ability to usher what's left of a once-bold agenda through Congress. The president's chance to confirm a Supreme Court nominee, a hoped-for reset for the administration and the party, could be dangerously at risk if Democrats are unable to count on their majority to overcome hardening Republican opposition.

Already, routine Senate business was being rearranged Wednesday, as the Senate Commerce Committee announced it would be postponing consideration of some of Biden's executive branch nominees because the panel, on which Luján is a member, needs all Democrats for the votes.

More pressing, though, is the upcoming Supreme Court confirmation battle to replace retiring Justice Stephen Breyer. Democrats have been eager to shift to the high court fight, believing it will galvanize voters at a time when Biden's broader legislative agenda, including his sweeping Build Back Better Act and voting legislation, have collapsed.

Sen. Dick Durbin, D-Ill., the chairman of the Judiciary Committee, said the panel is planning to push ahead with consideration of Biden's Supreme Court pick as soon as the president announces his nominee, expected later this month.

"We don't anticipate any difficulties," Durbin told reporters at the Capitol.

The Senate is split 50-50, with Democrats holding an ever-so-fragile majority because Vice President Kamala Harris can cast a tie-breaking vote.

Already Biden's agenda has fallen apart on Capitol Hill, taken down by the one-two punch of Republican opposition and two Democrats, Sens. Joe Manchin of West Virginia and Kyrsten Sinema of Arizona, who have joined Republicans to halt the president's priorities.

The ongoing COVID-19 crisis has stymied both parties, as senators have been forced to isolate after testing positive for the virus or being exposed. This week, two Republicans, Sens. Mitt Romney of Utah and John Hoeven of North Dakota, are working remotely because of positive virus tests.

Lujan's illness is a reminder it's not just Manchin, Sinema and Republican opposition, but the health and welfare of every single senator that could make or break the Democrats' hold on power and the outcome of Biden's agenda.

"We always knew a 50/50 Senate was going to require patience as well as cooperation and we hope he's back soon," said Sen. Tammy Baldwin, D-Wisc.

Sen. Ron Wyden, D-Ore., the chairman of the Finance Committee, said: "Everybody in the Senate can count so we all know what votes mean. But when I think about my friend, the senator from New Mexico, I just think about the times when with passion and eloquence he's been speaking for people who don't have clout, don't have power."

Past illnesses including strokes have led to prolonged absences in the Senate, most recently with Republican Sen. Mark Kirk of Illinois and earlier with Democratic Sen. Tim Johnson of South Dakota.

More recently, the Democratic majority was briefly at risk last year when Sen. Amy Klobuchar, D-Minn., underwent breast cancer treatment.

But Durbin indicated Luján's medical condition was not as severe, even though he acknowledged he had no information on when the first-term senator could return for voting.

Rarely has a president tried to accomplish so much in Congress with so slim a majority, and the fallout has been swift and stark.

Luján's office announced he checked himself into a hospital in Santa Fe on Thursday. His chief of staff, Carlos Sanchez, said the senator was then transferred to a hospital in Albuquerque for further evaluation.

His office added that Luján is still in the hospital but is expected to make a full recovery.

"Senator Luján was found to have suffered a stroke in the cerebellum, affecting his balance," the statement released Tuesday said. "As part of his treatment plan, he subsequently underwent decompressive surgery to ease swelling."