A “fifth column” is a group of “domestic actors who work to undermine the national interest, in cooperation with external rivals of the state.” This can be “overt or clandestine.”
There is a massive Russian fifth column in America which has until now, with some notable exceptions, tried to stay relatively clandestine.
But Tucker Carlson, by traveling to Moscow and giving Vladimir Putin a massive platform to lie to the American people directly, has signaled to the American fifth column that it is now time to shift gears to overt.