Democrats or Republicans get gifted a 535-0 majority in Congress. What happens?

I mean, there’s a little bit of a question as to whether they would go for remaining in power versus using power to enrich selves and backers. If they go for #2 first there’s a good chance the party implodes due to competing interests. If they go for #1 first there’s a good chance the party implodes due to strong demand for #2.

Look at State legislatures who have comfortable majorities this year, but also need to redistrict. Kent_Clark mentioned Missouri, but Florida Republicans also failed to accomplish much despite holding a solid trifecta - they had to hold two special legislative sessions and eventually gave up on redistricting due to bickering, opting to just use the governor’s plan.

~Max

If I can ask a slightly different version of the OP: what if one party was totally in control of all of the states while the other had all three branches of the Federal?

State conventions. See also the 21st Amendment.

~Max

Yep, putting a “D” or an “R” after the name of every legislator wouldn’t eliminate political and policy cleavages – it would just shift where they take place. You’d quickly have organized “wings” within the dominant party along regional or issue divides like Southern and Northern Democrats in the mid-20th Century and the splits in Republican Party in the late 19th Century over the gold standard.

Democrats instantly fracture into two factions.

Republicans: It doesn’t bear thinking about. But the rest of the world, with considerable justification, lobs every nuke they collectively control and turns the lower 48 into glass.

Democrats: Mandatory inter-racial or same sex marriage. Bonus tax breaks for both (.01%)

Republicans: Mandatory weapon ownership /w open carry.

The 2020 Republican Platform said essentially, “We support President Trump.” Can you say, “Cult of Personality?”

Dems: act like politicians, and get a little more accomplished than usual.

Republicans: act like ruthless, abhorrent, power crazed jerks who hate everybody but rich middle aged white Christian cis straight able-bodied American born men. And there would never be a meaningful election again.

…with a rider attached that legally defines any visible minority seen carrying a weapon in public as an “immanent threat”.

I’m going to agree with this. Without opposition, any party is going to fracture into multiple factions. They’ll likely eventually coalesce into two groups and then you’re back to having a two party system. Or possibly into three groups with a small third group as a potential ally of either of the two larger groups, depending on which one offers the best deal.