No dictatorships, no death squads, no theocracy -- my personal, plausible worst-case scenario, with minimal hyperbole, grounding things in actual past history and publicly-stated aspirations as much as possible:
The Supreme Court guts the Affordable Care Act, crippling President Obama's legacy, but stays the decision long enough to avoid immediate catastrophe in the insurance market and give Congress time to devise a "replacement" that basically returns things to the pre-2010 status quo.
Hillary Clinton wins the Democratic nomination, but triangulates on these and other issues too readily, rankling the base and faltering badly in her general election campaign under the weight of scandals both real and imagined. Meanwhile, Scott Walker unites various conservative factions and wages an ugly, divisive campaign.
With progressives demoralized, and moderates turned off by both candidates, energized Republicans sweep the 2016 elections, with complete control of the White House, a supermajority in both houses of Congress, and state offices nationwide.
Unlike the Obama/Reid/Pelosi trifecta of 2009, the Walker/McConnell/Boehner coalition is both more disciplined and more ruthless. The Senate filibuster is immediately neutered, and the Republican coalition routinely picks off handfuls of Democratic votes on key issues.
Walker promptly signs into law legislation eviscerating public and private unions (organizations that, despite their flaws, offered one of the last true counterbalances against corporate power, as well as financially underpinning the Democratic Party). This is cemented by the appointment of multiple Supreme Court justices that lock in a conservative, pro-business majority on the bench for a generation, making decisions like Citizens United, McCutcheon v. FEC, and the VRA invalidation routine. This is followed up by the Ryan budget on steroids: a broad-based rollback in federal programs and regulations since the New Deal (EPA, minimum wage, FCC net neutrality regs, whole cabinet departments), severe cuts in social programs, tort reform that shields abusive companies from lawsuits, tax reform that virtually eliminates rates on the very wealthiest (by eliminating capital gains, corporate and estate taxes) while increasing it on the poorest ("broadening the base").
Gerrymandering and changes to electoral college rules in key states deeply entrenches the party, insulating their majorities from any electoral blowback.
Oh, and we likely declare war on Iran, which makes Iraq look like a walk in the park.
tl;dr: A radicalized, uncompromising GOP wins complete control of all levers of government long enough to muscle through an ambitious Randian agenda alongside unprecedented structural changes that make it almost impossible to dislodge them from power. The moderating check-and-balance of the two-party system is seriously weakened, and federal policy tilts overwhelmingly toward wealthy and corporate interests at the expense of the average citizen's welfare.