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.