Two of the most common arguments used by Republicans and other defenders of President Trump during the impeachment inquiry are ones that are entirely within their own control.

The first is the claim that testimony presented in the inquiry should be dismissed because it’s secondhand. There are a number of problems with that assertion, including that some testimony described as secondhand is, in fact, firsthand observation of questionable conversations. What’s more, a significant amount of evidence in the inquiry is firsthand, including testimony from the ambassador to the European Union, Gordon Sondland, and text messages turned over to investigators.

The most significant problem with that claim, though, is that it rewards Trump’s apparent strategy of filtering his intentions in Ukraine through trusted deputies — who are then prevented from offering testimony. Acting White House chief of staff Mick Mulvaney is blocked from testifying because of executive privilege, which Trump could waive. Trump’s personal attorney Rudolph W. Giuliani could also testify, but Trump would have to waive attorney-client privilege. That these key witnesses aren’t appearing before investigators to offer firsthand evidence is a function of Trump, not of failure on the part of Democrats.

The other argument Republicans are using to diminish the inquiry is that it’s a partisan exercise, supported by most Democrats but no Republicans. That’s true, as the House vote last month on the inquiry rules documented. But it’s also true that Republicans have drawn a clear line in the sand for their caucus, making supporting the inquiry an unlikely proposition. There once was a House Republican who called for Trump’s impeachment: Rep. Justin Amash of Michigan. Facing backlash, he eventually left the party. He voted in favor of the inquiry rules.

Partisanship is, in fact, a central factor in the impeachment fight. It seems likely, given the composition of the House, that Trump will be impeached. The fight will then move to the Senate, where a two-thirds majority is needed to oust him — but such a majority would require at least 20 Senate Republicans to support ousting a Republican president. That’s unlikely for a lot of reasons, including that Congress is more polarized right now than it was in any of the prior impeachment efforts.

There have been three significant prior efforts to impeach a president. In 1868, Andrew Johnson was impeached but not removed from office. In 1974, Richard Nixon resigned before facing almost certain impeachment and possible ouster. Twenty-one years ago, it was Bill Clinton’s turn; his fate was the same as Johnson’s.

Despite the more than 150 years separating the first impeachment with Trump’s, we can make some observations about how partisanship in Congress has changed. The Voteview project, now housed at the University of California at Los Angeles, calculates ideology for members of the House and Senate in each Congress, allowing us to get a sense of how divergent views were in each of the prior impeachments — and how they compare with now.

In short: Congress is more ideologically polarized now than it was for Johnson, Nixon or Clinton.

When Johnson was impeached — shortly after the Republican Party became a significant political force — the ideological gap between Republicans and Democrats in both the House and Senate was 0.68 points. (These numbers are represented by the dots on the lines on the bottom set of charts above.) For Nixon, the divide was slightly narrower. For Clinton, there was a much wider gap in the House — 0.78 points — than in the Senate, where the gap was 0.68 points. (Much of the credit for that difference goes to House Speaker Newt Gingrich.)

Right now, the partisan ideological gap in the House is 0.88 points. In the Senate it’s 0.84 points.

Party control is important here. Clinton was impeached by a Republican-controlled House, for example, but a Republican-controlled Senate couldn’t muster the two-thirds majority to oust him. Trump is even luckier in that regard than Clinton.

He’s also lucky because it’s his party that has driven much of the polarization divide. The Democratic House and Senate caucuses aren’t much more ideologically liberal than they were when Nixon was under fire. The Republicans, though, have gotten much more conservative.

The causes and effects of the ideological gulf in Congress have been explored any number of times in any number of places. While there is a difference between ideological polarization and partisan polarization, it’s clear that the increase in partisan polarization correlates to the gap between the parties in Congress. (A recent study from Pew Research found that partisan polarization has also increased.)

Polarization isn’t destiny. Republicans on Capitol Hill and in the electorate could decide that what Trump has done — even if it was learned about secondhand — has made his presidency untenable. That seems unlikely, though, with a polarized Congress poised to see a sharp partisan division on the question of impeachment.

The net effect, though, is that Trump gets to use partisanship as both a sword and a shield. Partisanship means that he’s almost certainly not going to be removed from office, and it also means that he can disparage the impeachment as unfair and political.

Partisanship probably helped Trump win the presidency, with Republicans who were lukewarm on his candidacy eventually voting for him out of party loyalty, making the difference in a close race. Partisanship seems likely to also help him keep it.