>If Congress had a full Republican majority during Nixon’s years, he would not have been impeached.
That's at best "unclear". Attitudes were different, and there is some evidence of principled intentions even by the Republicans. If I were pressed for an answer, I'd say that the Republicans would have impeached, just weeks later than the Democrats. But, during that era Congress still thought itself coequal to the presidency and wanted to preserve their own power, which might have had something to do with that too.
>If Congress had a full Democratic majority during Clinton’s years, he would not have been impeached.
Which is funny if you ask me. They still defend him to this day, despite the fact that he opened the presidency up to extortion by any intelligence service competent enough to have caught on to his behavior.