Why can't active NASA astronauts endorse US presidential candidates?

Four people in red, white and blue shirts float in zero gravity in a cramped laboratory filled with wires and computers.

Today is election day. All U.S. citizens, whether at home or abroad, are encouraged to vote today — even those citizens in space.

Yes, NASA astronauts on the International Space Station (ISS) or on other spaceflight missions have voted in U.S. elections throughout the agency's history, and this year is no exception. The four NASA astronauts currently on the ISS have either already voted or are able to vote through casting special absentee ballots using NASA's Near Space Network network of satellites and ground stations. And because most NASA astronauts live in Texas near the agency's Johnson Space Center in Houston, Texas passed a law in 1997 officially allowing astronauts to vote from space, according to NASA.