The resignation of three top names from Stone's case signals clearly that the federal prosecutors involved believe Barr has acted with corrupt intent in reaching down to order a reduced sentencing ask. They, too, should be subpoenaed immediately so that they may describe the situation that led to their self-removal. This is an extraordinary situation: Even if a teetering-toward-fascism Republican Party will not so much as pretend at "concern" for Trump and Barr's move, the House can expose it without their assistance.
It must be done. The Department of Justice has within the span of days been turned formally into a tool for "processing" allegations against Trump's political opponents and, now, overriding the sentencing of Trump's criminal allies. Barr will resist, but it must be done regardless.
Sen. Chuck Schumer is calling for the inspector general at the Department of Justice to “open an investigation immediately.” That internal review, however, would be obviously insufficient. Rep. Adam Schiff, chair of the House Intelligence Committee and lead House manager in Trump's Senate-stifled impeachment trial, sent out an initial statement calling the reported Barr-Trump intervention a "blatant abuse of power" and, coupled with Trump's retaliatory firings, "the gravest threat to the rule of law in America in a generation." But it does not yet threaten subpoenas.
Schiff is correct in his diagnosis—and now the House must once again act. It is not optional; it cannot be danced around. Barr has committed an act of corruption so blatant that prosecutors have resigned rather than carry it out; he must explain himself under oath.