“Let’s be clear,” wrote Paul Krugman on Twitter at the conclusion of last night’s vituperative presidential debate, “a candidate for president just promised to put his opponent in jail if he wins. Everything else is secondary.”Trump’s promise to prosecute Hillary Clinton for her alleged wrongdoing during her tenure as Secretary of State has dominated much of the post-debate commentary, with leading center-left opinion makers declaring it an unconscionable breach of liberal-democratic norms. And indeed, there is no question that Donald Trump has corroded those norms throughout his campaign, and that his pointed remarks on Sunday night represented a continuation of that process. But the political media screaming bloody murder over Trump’s provocation has an important role in upholding liberal-democratic norms as well, and it cannot do that if it defines them selectively. And that seems to be what is happening here.Many of the figures now vigorously denouncing Trump’s threat to launch a politicized investigation of his political opponent should he win called for President Obama to do the very same thing in the early years of his administration. In fact, in 2009, just before President Obama’s inauguration, Paul Krugman himself called for the new president to launch wide-ranging legal campaign against former Republican officials, scrutinizing everything from interrogations to civil service appointments to its handling of voting rights. “If we don’t have an inquest into what happened during the Bush years,” Krugman wrote, “this means that those who hold power are indeed above the law because they don’t face any consequences if they abuse their power.” This position was popular across broad swathes of the Left at the time.There are two defenses of the Left’s apparent inconsistency on this question (the view that Bush administration officials should have been tried and convicted by the Obama administration, but that Trump’s suggestion that Clinton deserves the same is an existential threat to the democratic order). The first is that Hillary Clinton’s actions as Secretary of State were far less severe than the alleged crimes of the Bush years. This is a legitimate and arguable point. But it also amounts to a concession that it is acceptable for new presidents to prosecute members of the former administration if partisans deem the wrongdoing to be severe. In that case, Trump’s comments last night are less a norm-shattering violation than an expression of disagreement with the official Democratic view that the various Clinton scandals don’t amount to anything.The second response is that President Obama himself never actually threatened Bush officials with prosecution (although he did weigh the possibility of torture prosecutions early in the campaign). Ultimately, President Obama disappointed many of his liberal allies by declining to launch a probe into Bush administration wrongdoing. He deserves great credit for doing so—once again, the norm against such partisan inquisitions exists for a reason. So there is no argument that different standards are being applied to Barack Obama on the one hand and Donald Trump on the other. Rather, the problem is that the media referees who hastily declared Donald Trump’s threat outside the bounds of acceptable discourse participated in a full-court press to urge Barack Obama to do much the same thing. This kind of selective rule-setting by media elites undermines trust in the press and is itself corrosive to our democracy.None of this means that what transpired last night was not disturbing. As Jonathan Chait has aptly put it, norm violations occur “not all at once, but step by step, in a series of incremental, leapfrogging violations by the opposing sides.” Thanks to Trump, the convention that winning parties don’t use the executive to prosecute political opponents is weaker than before, and pundits like Paul Krugman are right to draw attention to this alarming development. But they shouldn’t pretend that they played no part in getting us here.