Bridenstine, an Oklahoma congressman and Navy Reserve pilot, encountered a brief setback when senators deadlocked, 49-49, on a procedural vote. Vice President Mike Pence was away and unable to break the tie. Arizona Republican Jeff Flake initially voted against, then changed his vote, allowing Bridenstine’s nomination to advance 50-48.

A final vote is expected Thursday.

Space historian Roger Launius says Bridenstine’s nomination was the closest vote and most contentious in NASA’s nearly 60 years.

Florida Democrat Bill Nelson criticized Bridenstine as one of the most divisive politicians. Democrats also blasted Bridenstine’s lack of space experience and past rejection of mainstream climate change science.