Republican senators have been left baffled and frustrated by Sen. Cory Booker’s unprecedented plans to testify against Sen. Jeff Sessions, R-Ala., a fellow sitting senator who’s in line to be the next U.S. attorney general under President-elect Trump.

Booker, D-N.J., is set to oppose Sessions in a Wednesday hearing, and while Republicans are allowing him to speak, it’s clear they’re not happy about it.