Democratic Sen. Tim Johnson underwent surgery after being hospitalized with symptoms of a stroke just weeks before his party, with only a one-vote majority, was to take control of the Senate.

The nature of the surgery or the South Dakota lawmaker’s condition was not known early Thursday.

There was no formal announcement of the operation, which lasted past midnight Wednesday and was disclosed by an official who spoke on condition of anonymity, citing the sensitive nature of the subject.

Apart from the risk to his health, Johnson’s illness carried political ramifications. Democrats emerged from last month’s elections with a 51-49 Senate majority. If he were forced to relinquish his seat, a replacement would be named by South Dakota’s GOP Gov. Mike Rounds.

A Republican appointee would create a 50-50 tie, and allow the GOP to retain Senate control.