Story highlights Sen. Thad Cochran's absence could derail a budget vote this week

Republicans hold a narrow 52-48 majority in the Senate

(CNN) Mississippi Republican Sen. Thad Cochran remains ill and won't return to his Senate duties Monday as was expected, his office announced.

The 79-year-old's absence raises doubts about the ability of GOP leaders to pass a key budget resolution -- a vote is planned later this week -- that is needed to advance tax reform, a top legislative priority for Republicans and President Donald Trump.

"Mrs. Cochran informed me late Saturday night that Senator Cochran has developed another urinary tract infection," said the statement from Cochran's chief of staff Brad White. "After a day of monitoring his condition, and on the advice of his physicians and other health care professionals, Senator Cochran has postponed his return to Washington. He will continue his recuperation at home in Mississippi."

White did not say if Cochran might be back in time for the vote, which, while not formally scheduled, is expected to take place late Thursday night or very early Friday morning, but the senator already has been out for several weeks.

Asked if the vote might be postponed to give Cochran time to recover, Senate Majority Leader Mitch McConnell spokesman Don Stewart said he did not have any scheduling announcements to make.