Manchin said after reviewing her public statements and tweets, he would oppose her nomination to run the powerful White House office.
"It is about protecting a merit-based system from political corruption and unlawful retaliation," Sen. Tammy Duckworth said.
House Speaker Nancy Pelosi remained defiant Thursday after tearing up a copy of President Donald Trump's State of the Union address.
Senators spent Tuesday explaining their positions on impeachment ahead of a final vote Wednesday on whether acquit Trump or remove him from office.
