All historic impeachment votes were (edit: almost entirely) along partisan lines: Clinton, Trump, and Johnson.

Nixon was potentially going to be bipartisan, which is why he resigned first — so we’ll never know for sure.

> votes were (edit: almost entirely) along partisan lines

As I said (I thought clearly), I'm not talking about the final vote. I'm talking about the trial stuff that happens before it.

For example, whether any witnesses are called. In Trump's impeachment trial they decided witnesses weren't necessary (https://www.npr.org/2020/01/31/801589634/sen-alexander-expla...).

Also, when Clinton was impeached, it was a Republican-majority Senate impeaching a Democratic president. So not really comparable to when it's the same party for both as with Trump. Of course they went through all the steps in the trial for Clinton because that Senate wanted to impeach (and probably wanted as much damaging evidence to come to light as possible).

Not exactly true. In every trial in the senate, the guilty votes come exclusively from one side, but the not guilty votes are historically slightly more bipartisan. For both Clinton and Johnson, all the guilty votes came from republicans, while all senate democrats and a handful of republicans voted not guilty. Trump's impeachment trial was unique in that it was a straight party line vote. Nixon almost certainly would have been impeached (the Democrats had a large majority in the house), and a large number of republican senators were expected to vote guilty (which would have been unique). He may have been able to escape a conviction in the senate, but the evidence was pretty damning and the American public would have been furious. In some sense his resignation was the only way to salvage the Republican Party.

Trump's wasn't by partisan lines, Romney voted for it.

You are correct, but at 48/52 with his vote joining the democrats it’s as close to partisan as one can get without it being complete.