Even for President Donald Trump, "I'll impeach you!" is an idiotic comeback to House Democrats who are pursuing an impeachment inquiry. To state the obvious, lawmakers don't get impeached, but if they did, I'd be willing to give up one of them to get rid of Trump (sorry, but I am certain the Democrats understand). The "No puppet, you're the puppet" game simply does not work in the current situation.
In the past, if Trump were to say the constitutionally proscribed impeachment process is a "coup," or that our allies will somehow uncover evidence contradicting the unanimous conclusion of U.S. intelligence officials that Russia interfered in our 2016 election the media might respond, "By coup, the president must really mean . . ." or "while most experts believe Russia interfered with the election . . ." Those days of contrived "balance" and deference to a corrupt president are gone.
Mainstream outlets have collectively decided that Trump is now a threat to democratic elections, so it is time to get serious and drop the pretense that there is a legitimate "side" (e.g., Ukraine has the DNC server! Asking China to interfere in our elections is fine, just fine!) whom Trump, Fox News and the Trump cultists represent.
As media organizations knock down the Potemkin village of normalcy they had erected around Trump, nearly all but the Trump dead-enders (e.g., Reps. Devin Nunes of California, Jim Jordan of Ohio) seem unwilling to defend Trump's impeachable actions. They've either gently called him out, as Sens. Susan Collins, R-Maine, and Rob Portman, R-Ohio, did, or tried to pretend it did not happen (It was a joke! Just joking!). You are not getting a whole lot of Republicans willing to condone foreign countries picking our presidents.
One can conclude that Trump finally "went too far," but there are other factors at play that distinguish this situation from past crises.
For starters, Trump's crazier-than-usual reaction (borne of panic) and utter lack of message coordination have left most Republicans unable to help him, even if they wanted to. When they are rather mute and the media won't give equal time to crazy talk, the truth has a way of bubbling up.
Second, as we saw with the trade war and with the Kurds in Syria, the president's awful and inept policy choices are coming home to roost. International chaos, a humiliating and disastrous sellout of the Kurds and an economic slowdown prompted by a trade war are really pushing Republicans to consider whether Trump's survival is in their best interests.
Third, the commencement of formal impeachment proceedings certainly adds a level of gravity to events and has had a corresponding effect of wigging Trump out. The swift movement in the polls in support of impeachment seems to have stunned Republicans and has disabled their usual confidence that spin will save them.