Vol. 8: The Modern Society (published 2018) is the last book in the series [despite calls to cover the left-wing nationalist tendency] and deals with Democratic Choice, the left-wing coalition between the Radical Party, the Green Party and the Reform Party (pre-1999), and ultimately an unitary party, with the merger just managed to get in the book before it was released for publishing.
The Exchange led to millions dying due to nuclear blasts, and thousands more to anthrax attacks in places like New England, and America was angry at the Social Conservatives. In this, many turned to radical solutions offered by the Radical Party, but also by the American Union of Fascists and the theocratic Salvation Party, along with the hardline national-populist America First Party. The Green Party was originally established by Artemis Fowl, former Social Conservative vice-president and overall ambitious so-and-so. Shaken by the exchange, he shifted left in beliefs and argued for a radical environmentalist agenda, but he was too "establishment" for angry Americans and he sensed it too. Hence he went cap-in-hand to the Radicals and negotiated an united ticket and an electoral pact, the very beginning of Democratic Choice.
The Radical Tadeo Murillo won a victory in 1984 based on popular anger and a desire for serious change from the Social Conservatives, and he, well, he didn't really deliver. The economy, already at Great Depression levels from the Exchange, failed to recover while the America First controlled Congress obstructed Murillo's more socialist bills, leading to frustration from his side. And then the right-wing went too far. Governor Jim Buckley of Louisiana decided that Murillo was a fundamental danger to America and must be removed at once and America put under the control of a "National Salvation Government" led by General Alexander Freeman-Smalls (the 1984 America First nominee). The coup attempt failed, and Buckley doubled down by declaring national rebellion against the "socialist government". He was allied with Salvation Party leader and incredibly paranoid pastor Jack Stevenson who declared a "Holy Union of America" in the Plains. America, already dealing with the effects of a nuclear exchange and one of the worst depressions ever, now was thrown in civil war. And then if things couldn't get any worse, Murillo was assassinated by a NSG agent in 1987.
Handing power to the Green Arya Moon, who was inaugurated in the worst situation possible. Surprisingly enough for someone known to have taken hard drugs barely a few years before, she deftly dealt with the NSG and before the next election Jim Buckley was dead and the NSG reduced to a bunch of disunited terrorist cabals. The economy recovered due to her reformist policies, and she won a landslide in 1988, which she decided would be a mandate for a radical left-wing reform of the country which she dubbed her "Modern Society". This included universal healthcare, social security, a national living wage, disability rights, and perhaps most consequential of all, a constitutional convention that reformed America into a semi-presidential system with a First Secretary as a Prime Minister.
Of course, all this success can't go by without two foreign policy controversies. The first was the trade deal with Qatar, now taken over by an ultra-libertarian hippie cult, that many now allege was the reason their party in America endorsed her in 1988 and 1992 and supported her party in congressional coalitions despite it making absolutely no ideological sense. The trade deal included a fair few "investments" in the Qatari economy that essentially and ironically made the libertarian "utopia" a state-funded experiment heavily reliant on United States dollars.
The second foreign policy controversy would be the most damaging to President Moon. Eastern Europe was heavily damaged by the Exchange and radical German supremacist organisations seeking to wipe out non-Germans were on the rise, so President Moon got Congress' authorisation for military intervention. Acutely aware that it could prove her downfall, she managed the intervention half-heartedly and allowed the radical supremacists to push away American troops and make the situation even worse than it ever was. With coffins returning, the American people decided to call time on Arya Moon and deny her a third term [which she was eligible for, technically] in 1992.
With the Democratic Choice coalition now out of power, they rallied behind non-interventionism and opposed Moon's successor who betrayed her promise of isolationism in favour of amping the war up and "winning it for good". In the end, former Reformist and now Green Patty Allen won the Presidency in 1996 and promptly withdrew troops from Eastern Europe, which Americans decided was too much of a hassle to deal with. Patty Allen was a firm social-liberal and without Moon's hang-ups about going too liberal when she thought she needed to be a national unifying figure, and so she implemented wide-spreading social reforms and oversaw the economy recover more and more.
Then the Canadians invaded.
The 2000 election proved a strong Democratic Choice victory thanks to the Canadian War turning into the Americans' favour just in time for Allen. She oversaw the economy weaken in her second term, which many attested to it being a boom for too long, but it wasn't a severe recession at least. The Democrats won 2004 with General Robert Cermak, a Green, who won in a landslide. Clearly, there was a permanent Democratic majority!
And then the Democrats fell into infighting, Cermak left the party to run independent and a Radical was nominated for the first time which led to DC losing 2008 to the moderate-conservative Rally for the Republic. In opposition for the first time in 12 years, the Democrats rallied the best they could and under the Radical John Malone, the Democrats adopted a new platform, that of firmly socially-liberal socialism, not the old Green socially-liberal economic-reformism nor the old Radical socially-conservative economic-socialism. And that led the two to increasingly consider a merger.
By the 2016 election, there was still a distinction between a Radical and a Green, but it was a difference of shade, not of kind. Najat Belkacem, their 2016 nominee, was a firm "Moonite", strongly believing in Moon's Modern Society. By 2018, the parties voted to merge as one to present a fully united alternative to President Bennett and Our Millennium.