Jeet Heer states what should be obvious: if Democrats try to impeach the president and fail, it will completely legitimize his presidency.
The practical problem is that for impeachment to be meaningful, Trump would not just have to be impeached by the House of Representatives (which requires a simple majority) but also removed by the Senate (requiring a two-thirds vote). It’s easy to imagine a scenario where the Democrats win the House of Representatives in 2018 and have the necessary votes for impeachment. But even in that best-case scenario, in which Democrats win every toss-up race for the Senate, they would still be well short of the votes they need in the Senate. Which means that kicking Trump out of the White House by necessity has to be a bipartisan effort with significant Republican buy-in.
The Republican Party has proven that they will tolerate just about anything from Trump. They continue to stand with him despite his demented tweeting, the political support he’s given to Roy Moore, his repeated expressions of contempt for the justice system, and his cavalier threats to launch a nuclear war. Unless Robert Mueller finds the possibly apocryphal “pee tape,” Republicans are likely to remain loyal to Trump. In fact, there’s a real possibility that even if the “pee tape” is real and widely viewed, Trump would still remain politically sacrosanct among his own party.
The most promising route for stopping Trump, then, is through the ballot box.
This means that we're probably stuck with him for 1142 more days, but if we win back the legislature, we can thwart him nearly completely.