Speaker Nancy Pelosi’s dramatic impeachment move is not an offensive one; it is a rearguard action. Democrats come to this not from a position of strength, but from one of necessity. Having to “put up or shut up,” they are now desperately trying to preserve their narrative of Trump’s presidency.

History shows that under normal circumstances, presidential impeachment is a losing proposition for its perpetrators. There have only been two in American history — Andrew Johnson a century and a half ago and Bill Clinton two decades ago. Both failed to remove their targets.

The one instance in which impeachment did provide leverage to remove a president underscores why the Democrats’ attempt will prove futile and likely counterproductive.

Nixon remains the Democrats’ nirvana. He resigned to avoid impeachment and virtually certain removal. Its lessons, however, are that overwhelming evidence and bipartisan consensus are required. With Trump, the Democrats have neither.

The Republicans’ lack of at least the latter during Clinton’s impeachment should be the ultimate warning to Democrats today. The Republicans’ effort did not simply fail; it also started their intended target’s rehabilitation. Clinton went from villain to victim to vindicated.

The Democrats’ position with Trump now is worse than the Republicans’ was then.

First, they lack the evidence to make an impeachment case beyond their base. For most Americans, the protracted Mueller investigation’s bland conclusion and Mueller’s poor testimony closed that case; pursuing it further looks less like evidence and more like vendetta. Their latest effort regarding Trump’s Ukraine phone call is even less developed, making their move appear all the more precipitous.

Second, today’s Democrats control only the House — and not overwhelmingly. This means that they could start the process of formally impeaching Trump, but without hope of Senate conviction.

Third, when Republicans moved against Bill Clinton, they were nearly united. To get to their sure defeat, today’s Democrats will cause deep fractures between their left and moderate members. Those fractures will translate into tough, and likely costly, votes for their moderates in both bodies.

For all these reasons, Republicans will hold, Democrats will split, and the result will be de facto bipartisan exoneration. With such an outcome, the American people will side with Trump, just as they did with Clinton.

So facing all this, why have Democrats chosen to proceed on a political suicide mission? They’re not choosing to do so as part of an offensive strategy; they’re being forced to for defensive reasons.

Those who’ve been pushing for an offensive strategy have long been out there on the left. The Democrats’ congressional leadership, however, successfully stymied this push, for all the reasons already mentioned. They knew better. Several vulnerabilities that have begun to emerge, though, have changed the dynamic.

A now obvious one is that Democrats will nominate a presidential candidate from the left — likely their most liberal ever. Still, this process will not be over quickly, and the longer it goes, the further the Democrats’ nominee will have to go left.

As the contest goes left, it will be increasingly likely that, as the Democrats’ shrinking field competes for votes there, the remaining candidates will play up impeachment that much more to secure the nomination. This external call will further strengthen those inside Congress already calling for it. It also magnifies the Democrats’ difficult predicament.

Having claimed since the inauguration that Trump deserves impeachment, with presidential standard-bearers amplifying the demand, and with Democrats’ House majority enabling impeachment proceedings to occur, how does the party explain not doing so? Failure will call into question the sincerity of their earlier charges.

Also, the further left the party goes, the more the party will be dependent on the Left’s support. Moderates’ qualms will be discounted. And the further left the party goes, the less likely the Left will be to get moderate support anyway. Direction will become destiny, and impeachment becomes one of its most obvious outcomes.

The Department of Justice’s own investigations into the Obama administration’s pursuit of Trump — the investigation of the investigators — perversely could also add to an impeachment push. Any evidence that the Obama administration’s investigations were politically motivated will dangerously undermine the narrative Democrats have nurtured from the beginning. In order to validate their version, Democrats could feel a need to commit to impeachment in desperate hope of sustaining this narrative.

Since before Trump took office, many Democrats have sought to remove him from it. Their charges have evolved over the last three years, but the verdict has remained the same: Impeachment. Recklessly swinging their impeachment hammer, they see everything as a nail. Trump’s Ukraine call is just these Democrats’ latest issue; it will not be their last.

Thus far, the pressure Democrat leaders have felt on impeachment is nothing compared to what is coming. Far from the offensive opportunity the Left sees, impeachment is something they’ve been pushed into by a defensive dilemma they can no longer ignore.