Why impeachment now?

The five reasons Republicans as well as Democrats are looking differently at the politics of Trump's impeachment. 

I suspect that the group of people who didn't favour impeaching and removing Donald Trump until they read the transcript of his July 25 phone conversation with Ukraine's president is a very small group indeed.

If that's so, why is the formal impeachment process happening now — and why are some Republicans, such as Francis Rooney in the House and Mitt Romney in the Senate, apparently open to it?

Political scientist Matt Glassman floats one theory: that "recent events have disrupted what was a pretty good bargain between GOP and Trump." That bargain, he explains, was this: "The GOP gets most policy control; Trump gets adulation, praise, and freedom from corruption investigation."

With Democrats now having a House majority, Republicans no longer have a legislative agenda, and therefore no longer benefit from the bargain (at least outside of the steady stream of judges Trump nominates and the Senate confirms).

Plausible! It does explain why Trump's occasional violation of party orthodoxy on issues such as trade or Syria looms larger now than it once did. 

I'll suggest a few more possible reasons for why things have changed. 

It's cumulative.  Impeachment is political. It's not like the law, where a defendant is innocent or guilty of each charge separately, and an accusation is worth an indictment or it isn't. The constitutional standard is vague, and deliberately so.

There's nothing wrong with or unusual about members of Congress who seem willing to live with example after example of abuse of power, only to eventually decide that this much is too much. After all, every president has probably done something that a number of members of Congress believe is an abuse of power. Perhaps the problem isn't so much that the Ukraine scandal is different, but that it's one more offense on top of the others. 

It's continuing. Several people have suggested this theory. It's one thing to decide to move on from focusing on misbehaviour that took place in the 2016 election; it's quite another, early in this cycle, to ignore attempts to improperly affect the coming election. This distinction seems to be particularly important for Democrats who were previously reluctant to risk a partisan impeachment. It appears to have invoked the politician's logic: We have to do something; impeachment is something; therefore, we must impeach.
 
It's the election cycle. As for the Republicans, they had strong incentives to show a united front from the point when Trump clinched the nomination in 2016 right up to the 2018 midterms. Those incentives, while not absent, are considerably weaker as we approach the end of 2019.

We've seen the effects. Trump has had to veto bills after some Republican senators joined Democrats to pass legislation he didn't want. The government shutdown ended in a rout as Republican senators stopped backing Trump's intransigent position on his border wall. And they've been quicker to criticize his policies, as they are doing with Syria and the Kurds.

Now, in the fall before the presidential election year, Republicans realize the window is closing. If they don't act soon, they're stuck with Trump's re-nomination, and then with the chance of a loss in November (as indicated in current polls, although there's plenty of time for that to change) or the possibility of having him around for another four years. They may decide that those risks are better than the risks of replacing him, but as of now it's still a choice. 

It's Mulvaney. After an inept transition and the first months with Reince Priebus as White House chief of staff, things improved when John Kelly took over the job in mid-2017. But Kelly eventually seemed to give up on trying to achieve a professional White House, and after he left at the end of 2018, new acting chief of staff Mick Mulvaney hasn't even tried.

With the administration more chaotic than ever, it's more likely that individual legislators of both parties are unhappier with Trump. It doesn't help that Mulvaney was one of the House radicals who has never shown the ability to work with the rest of the Republican Party, let alone with Democrats. Members of Congress don't vote to impeach and remove because the White House is badly run, but a badly run White House is more likely to turn everyone against the president.

So which one of these explains the shift in Washington? Probably all of them, at least somewhat. Or, to put it in a way that the political scientist Richard Neustadt might have phrased it: Trump has become such a weak president that he's now vulnerable to losing the job entirely.