The SCOTUS is an apolitical body, they get lifetime tenure specifically to ensure that exerting political pressure on them is not a huge concern. It's nonsensical to talk about partisanship when none of the justices are beholden to any political party. Last time a party *did exert political pressure* on the SCOTUS was under FDR, *it was the *Democrats*, and it was *with the threat of court-packing*, for the purposes of pushing through the New Deal without having to worry about the Supreme Court putting some of its provisions into question. Everybody can see what's happening here - some people may have short memories, I don't.the constitution does not limit the supreme court, so changes can be made. the court is supposed to be non-partisan, and if it becomes so, it needs to be re-balanced. If the political will to increase the court is there, it will happen.
Say it how it is - you don't like the fact that the court is split in favour of conservative values for the first time in decades, rather than in favour of progressive ones. That much is true, but it's not a matter of partisanship - justices are split on ideological grounds, not by party alignment. I wouldn't even say that the court is split to an extent that warrants concern either since Justice Roberts has proven to be a wild card. It's 5-4 at best now that Barret replaced Ginsburg.
The actual reason why this is a problem for the Democratic party is that they've gotten mighty comfortable with using the SCOTUS to enshrine "rights" that are not at all enshrined by the Constitution, as opposed to using the far more difficult path of amending the Constitution as necessary. Liberal-leaning judges tend to treat it as a "living document" and adhere to the "spirit of the law" or some such nonsense, conservative ones tend to be textualists who are only concerned by what is and what is not written. There's already a system in place that enables changes to the Constitution and the SCOTUS should've never been used to bypass it.
If the Democrats want to claim that the court became partisan, they will have to show some examples of cases where such partisanship was evident - Barret was only just confirmed and they're already talking about it like it's a done deal with zero evidence. As for the legality of the move, of course Congress can do it - I've outlined that it has in the past, on multiple occasions. I was saying that there are no signs that the court "needs fixing" and that such a move has no support amongst the citizens - it's 100% political and, ironically, partisan to do so. The Democratic party doesn't like the result of the game, so they're going to change the rules - same modus operandi as usual. They won't be answering for their actions to a tribunal, but they will be answering to the American people. I certainly hope they won't make that mistake. Last time they were warned that they'll regret acting recklessly with American institutions "sooner than they think", and I think McConnel proved that he was correct in his estimation.