The reasons why coalitions won't happen in the US congress is because its easier to seize power from the traditional party apparatus and use that to advance your cause as opposed to creating a new party. The Dems and Goopers aren't splintering. In fact they're coalescing around some basic points. GOP = Trump Humping. If Trump is for something, so are they lock stock and barrel. Dems are coalescing around expanded govt intervention in health care, college affordability, and corporate regulation paid for by higher taxes on the rich. Yes, they may disagree on details but they've thus far been remarkably united.
That's because the Ds have great cat herders in Pelosi and Schumer and the GOP doesn't.