The Founders were fearful that impeachment would become a partisan endeavor, and they were right to be fearful. It has. That’s why the Founders set the bar so high for a Senate conviction.
If you look back on the history of US presidential impeachments, you will find a great deal of partisanship in the support for impeachment and removal, except for the impeachment that didn’t happen: that of Nixon, whose opposition was so bipartisan that he realized Senate conviction was likely and he stepped down before the impeachment ever occurred.
You will also find that the majority of the bipartisanship and/or crossing of party lines was by Republicans rather than Democrats, and it was Republicans voting against impeachment and/or removal of a Democrat president. This will probably not be a surprise. Democrats tended much more to vote as a bloc for impeachment and/or removal of a Republican president.
Take a look at the vote on the impeachment of Democrat Andrew Johnson. Republicans held an enormous majority in the House, and there were only 4 defections out of 126 GOP House members voting “yea” to impeachment. Of the 47 Democrats, only 2 went against their party to vote “yea” instead of “nay.” So the House vote was highly partisan. However, in the Senate – also very strongly controlled by Republicans (45 to 9) – something quite different happened. Of the total of 45 Republicans, 10 voted for acquittal, which was just enough to acquit Johnson by a single vote.
Nixon I’ve already discussed, but for Clinton we had a House vote that was mixed on the different articles. On the first article (perjury to the grand jury), there were 5 crossovers from each party. On the second (perjury in the Jones case) there were also 5 Democratic crossovers but 28 on the GOP side, and so that measure failed. On the third (obstruction of justice) there were 5 Democratic crossovers and 12 GOP ones; the measure passed. On the fourth (abuse of power), there was 1 Democratic crossover to 81 Republicans who crossed over, and the measure failed.
In Clinton’s Senate trial, every single Democrat voted for acquittal on both counts. Even if every Republican had voted to convict, there would not have been a 2/3 majority to remove Clinton. But 10 Republican senators voted against the first article and 5 voted against the second, a bipartisan vote on the GOP side only. This resulted in the first article not even getting a majority, and the second only getting a tie vote.
Interesting, no?
Which brings us to the recent impeachment of Trump. In the House, no Republican voted for either article, and only 2 Democrats crossed lines on the first article and 3 on the second. Extremely partisan. And I don’t even have to link to the Senate vote, because it’s easy to remember there was only one crossover, Mitt Romney. Almost completely partisan, the most partisan in the history of US impeachment trials.
Which indicates another interesting point: impeachment may happen again and again, since the bar is so low. But as far as conviction goes, if Democrats ever control the Senate by 2/3 and there is a Republican president, he or she may stand a good chance of being removed on a party-line vote. But if the GOP ever gets control of that much of the Senate and there is a Democratic president, removal would be less likely, at least if you look at the historical precedent.
However, at this point things have gotten so polarized that it also could happen. All bets are off.
Fortunately, that sort of imbalance hasn’t occurred in recent decades. During FDR’s tenure the Democrats had huge Senate majorities, but of course FDR was a Democrat as well. The same was true for Lyndon Johnson.
One can only conclude that the Founders knew what they were doing in setting so high a bar. Of course, that doesn’t stop the sort of stunts that the Democrats pulled this time, impeaching because they could do it and because they thought it would help them politically despite the fact that they would not and really could not secure removal.