The people need something to believe in

When a movement is able to reverse itself on policy without losing adherents, when a movement is able to attack the institutions of the state without the state's being able to retaliate or to eliminate the movement's leadership, when a movement has infiltrated the state's military forces and thereby disabled them from executing the orders of the state's leaders, when a movement is able to over-rule the dictates of the party orthodoxies by dividing the parties into pro-movement factions and anti-movement factions then the nation has given up its organization as a functioning state where the institutions of the church constitute a separate dimension of social order. 

Instead the nation has become an instrument of the movement where the feelings of patriotism and the feelings of spiritual submission to a higher power have become fused into a single-minded dedication to the dictates of the movement's leadership. Then the nation becomes ripe for dictatorship and, ultimately, for totalitarianism. 

Totalitarianism is a form of national spiritual collapse. It is a collapse that comes of the parties (that had formerly constituted the legitimate authority of the state) losing their impulse; the parties (that conducted the affairs of state in the legislatures) becoming vacuous and spiritually hollowed out. 

The movements replace the institutions of the state and become the only institutions that have a robust national spiritual sensibility. Movements tend to be able to over-ride party politicians because the parties lack spiritual impulse, a spiritual impulse the churches lack as well. Only the movements have enough spiritual drive to captivate the populace. 

The people need something to believe in and the movements give it to them.