This is my humble prediction for the future of US politics. It is based on a comment I wrote here.
Sometime next year Mueller will return his report on Donald Trump. I suspect it will focus more on Obstruction of Justice then collusion with Russia but will trigger the impeachment process.
While all Democrats will support it but Republicans will be split. The establishment Republicans, tired of Trump’s antics sabotaging policy implementation and his unpopularity reducing their chances of being reelected, will support impeachment. The Tea Party Republicans will support Trump not so much because he supports their beliefs, but because they are mostly authoritarian followers, who support leaders just because they are leaders.
After Trump is removed the anger between the two Republican factions will be so severe the party will formally split. There will be three major parties in Congress and three major parties competing in elections. Once Trump is out of power he will be quickly abandoned by the Tea Party.
With the right-wing vote split, it will give Democrats a huge advantage in elections. They may even to gain majorities with only 40% of the vote, a common event in multiparty democracies that use first-past-the-post.
Freed from the Trump and the Tea Party the mainstream Republicans will become move closer to the center, especially on social issues. As that happens fiscal conservatives will be drawn out of the Democrats and they will become more left-wing. With their majorities, they will implement a significant expansion of the social safety net.
At first, the Tea Party will be more popular than the establishment Republicans, but over the years their popularity will decline as they fail to achieve any of their goals. As they decline they will become more radical in the hope it will boost their popularity but will only make it worse. Within about 8 years, they will disappear entirely from Congress and the Republicans will be back to parity with the Democrats but will have largely shed its extremists and become more of a typical right-wing party.