Actually it does qualify as a legal threat...
There are three elements of the offense of making an illegal threat:
(i) there must be a transmission in interstate commerce [Schumer’s words were transmitted by video, radio, blogs].
(ii) there must be a communication containing the threat [Schumer in front of microphone and cameras].
(iii) and the threat must be a threat to injure the person of another:
“You have released the whirlwind, and you will pay the price. You will not know what hit you if you go forward with these awful decisions.” -Chuck Schumer 3/4/2020
What would “you would pay the price” can a sitting senator do to a sitting Supreme Court Justice? “You will not know what hit you” IS A THREAT and “if you go forward with these awful decisions” is obstruction of justice.
Threats can also sometimes be punished under the statutes criminalizing assaulting, resisting, or impeding certain United States Government officers or employees or assassinating, kidnapping, and assaulting government officials of the United States.
When a threat is made against a judge, it can be considered obstruction of justice [18 U.S.C. § 1503].
“Pay the price” could mean nothing more than “You will get nasty letters” if it came to a court defense.