On the pretext of a bogus emergency, Trump sends a large contingent of regular troops and red state National Guard members to cities in blue and purple states. They arrest the local political and intellectual leaders, who are ultimately sent to Third World prisons; reliable reactionaries are appointed to replace them. Unfriendly TV stations are taken over and left-leaning newspapers and universities are closed. The airports are put under federal control. Demonstrators are arrested or shot on site. In this way, dissent is completely quelled.
The internet is brought under control on a federal level, since it doesn’t recognize state boundaries, and Democrats in Washington are imprisoned, but for the most part, the red states are left alone. Some red state governors follow Trump’s example and imprison government critics; others continue to tolerate some level of criticism. Federal control increases as dissenters flee to the red states, but it never reaches the level that is experienced in blue states.
That’s all it takes, folks. And it would probably be blessed by the Supreme Court. This is why the Court’s ultimate decision on the scope of Trump’s emergency powers is so important.