> This basically put all the secular parties (both left and right) in a bind.

Which is the solution. Bind themselves and everybody else.

Have a constitution that enshrines the right to things like fair elections and free speech. Have an amendment process that requires two thirds of the Senate, in a Senate that only has one third of the seats up for election in any given year. And three quarters of the states, which rarely agree on anything. Things like that.

Then even if the populists get in once, they don't have the power to end elections or free speech. And once the people see their true nature, they lose the next one.

Obviously you do have to be very careful not to allow any end runs around that process.