How does an autocrat claim power legitimately in a democracy: watch Trump do it.
Ever think that Trump does not want a working Congress? The Congress is the one power center that can actually remove him from office. It is absolutely necessary for Trump to break its power. The other power centers, like the media and the judiciary and the intelligence community Trump frames as enemies and can delegitimatize. The Congress is different. They helped him get elected and they are legitimatizing his administration. So what does he do? He makes sure that they are unable to govern. He has stepped on every effort the Republicans have made to pass legislation and then, he blames them.
Yet the Republicans, who have been living in a fantasy world about ACA, tax reform, LGBT rights, are still in the fantasy land that Trump is the leader of their party and that he is actually trying to pass their agenda.
First Trump breaks their power, then he will require them to pass legislation that he will get credit for.
Every day Trump is eviserating our democracy, working toward a one-party system that will be under his control. All by legitimate means.