> to force all states to go along with it you definitely need an amendment

Not necessarily — Congress has vast powers under the Fourteenth Amendment. See the just-published "Abolish the Senate" by lawyer and well-regarded liberal commentator Thomas Geohegan [0].

[0] https://thebaffler.com/salvos/abolish-the-senate-geoghegan

Good luck finding a Supreme Court that would go along with it.

It's been suggested that Congress could protect any given enactment by expressly stripping the Supreme Court and lower courts of jurisdiction to review the enactment's constitutionality — which Congress seemingly has plenary power to do under Article III, section 2, clause 2. [1]

[1] https://newrepublic.com/article/158992/biden-trump-supreme-c...

This is correct. Congress can severely curtail the power of the Supreme Court if it wishes.

This sounds like a precedent that's bad to establish because it cuts both ways.

Precedent and norms have somewhat gone by the wayside in recent years, so that might not be a compelling reason not to act.

Given that Republicans removing the filibuster for Supreme court nominations was preceded by Democrats removing the filibuster for lower court nominations and Cabinet appointees, we already have plenty of evidence of one side doing things for convenience that end up blowing up in their face later on.

The response to the slippery slope is not to keep on sliding.

You probably don't; a federal law is probably sufficient.

Article I section 2 is the relevant part of the Constitution; it makes absolutely no statements about "The states are free to choose their way of allocating their House representatives so long as they are directly elected" and indeed that is absolutely not true; in fact the restriction to single member districts is a federal law passed by Congress, 2 USC 1 § 2c [1], after gerrymandering was producing multimember districts that were not state-wide in order to disenfrancise minorities.

[1] https://www.law.cornell.edu/uscode/text/2/2c