First of all, it should be noted that if a Senator legitimately believes a judicial candidate is not sane, that would be a completely valid reason to reject him as a candidate. (There was also a slightly less severe position that Rutledge had demonstrated recent behavior which put in question his general judgement which could impact his effectiveness as a judge rather than it simply being a matter of him expressing political opinions.) There also was evidence of Rutledge suffering from issues with alcoholism by the point he was rejected by the Senate, which also would be a completely legitimate reason to block his nomination to such an important post.
Ultimately, even if you argue the Rutledge rejection was purely about politics, and believe its fine for the Senate to consider the politics of a candidate, that's extremely different than the blanket rejection of any nomination from a President of the other party. (It was also understood until at least extremely recently that the political angle if valid only applied to a candidate with extreme positions and not merely views they disagreed with rather than the Senate only accepting candidates they would view as ideal when a President of another party is in office.)
When you get down to it, there is not much difference between a political party blocking a nominee for basically a whole year (probably literally a year until a new nominee is confirmed if the plan works) and a party saying they simply won't confirm an elected President of the other party's nominees to the Supreme Court period.