Sorry for not getting to this question sooner.
And the six was more referring to how many party systems we have, like we are on number 6 in OTL and number 7 is likely coming in several years, but will be f*cking rough time there.
ITTL, Number 7 will be established by the mid 2020s or so after large refoms are pushed in. One would be for the dissolution of electoral college and replacng the US voting system with either instant runoff voting or STAR voting becoming the replacement for it. Another would be the implementation of term limits on Reps, Senators, and Justices.
I could see this very much being in Sanders' 2nd term when they would get eough governor support and so on to implement on all levels. By 2020, I imagine the Progressives have firmly caught the Democrat party and part of a coalition to force the two end as the old guard would be fighting tooth and nail, if they haven't retired. By 2022, it's p[retty much the beginning of the transitional phase to the new system. By the late 2020s, smoke will clear and things settle.
With the term limits and so on, this makes it open season and with the generation gap, we could see young politicians flocking to new parties and making them the big ones. Here would be the like new major political parties, all of these existing ITTL & OTL:
The Big Three
American Solidarity Party- A Christian democratic party, this would become the new main party for various political conservatives. They would function very much like plenty of Christian democratic parties around the world, especially in Europe, though likely to be influenced by Agrarianism and modern American romanticism. Also becoming increasingly aligned with the mainstream Protestant churches instead of the evangelicals. Center-right.
The Unity Party- A centralist party and where alot of the heirs of the old guard and the like fall into. Very much in a way kinda like the moderates of old and try to put themselves up as kind of middle path to take. Probably won't be that popular for a while though still pretty good in purple states. Would also likely absorb the Reform Party in the process. Center.
Justice Party- The new major party for those in the center-left to left, would be a social democratic party and would be inspired by alot of the values and pursuits over of Bernie Sanders. AOC and "the Squad" and all the progressives are now chilling here. Probably the pretty popular ones for a bit of a while though still arguably not as ambitious as some may desire. Center-left.
Other Notable Parties
Green Party- The same old Green Party we all know and love. Still very much focused on the environment through they also tend to be syncretic over the political spectrum in how to get there. Still pretty leftist though and it still could have some rather questionable people at times. Home to center and leftist survivalists. Green.
Freedom Socialist Party- Where alot of the more hard lefties are going. Talking about the people who would wanna resurrect Project Cybersyn, make worker co-ops and the like mandatory along with plenty of other rather ambitious ideas that would intimidate more than a fair bit of politicians. Likely where alot of the leftist gun enthusiasts and the like hand around with. Having increasingly close ties with the US Transhumanist Party. Far-left.
Pirate Party- Part of the global Pirate movement and likely still be around. Increasingly affected by memers with a hackvisit streak. Still around. Pirate politics.
Constitution Party- Yes, they are still around unfortunately though not as crazy as OTL if for pragmatic reasons. Got the scraps and leftovers from thsoe who weren't allowed into the Unity Party. Right.
Libretarian Party- Somehow still around despite being viewed as much of a relic as the other two were. More left-libretarian now than OTL and has become the de-facto home of old school anarchists since no one else would take them. Center/Center-left libretarian.