Democrats swept into Congress this year with a promise to hold Donald Trump’s feet to the fire in a way Republicans have so far been unwilling to do. But already, they seem to be butting up against the limits of their Congressional powers as the president refuses to comply with their probes. On Wednesday, the White House informed the House Oversight Committee that Stephen Miller, Trump’s anti-immigration policy ghoul, would not be made available to testify, citing “long-standing precedent.”

The strong-arm tactics aren’t surprising, coming from a notoriously confrontational president. They have, however, left Democrats searching for recourse. “There is no clear, simple, easy enforcement mechanism because it has depended on institutional relationships for so long,” John Bies, a former Department of Justice lawyer, told CNN on Thursday. “The system has never really been tested this way.”

Congress, in other words, has a mandate to serve as a check on the executive branch, but that authority is largely dependent on subpoena power. What if the subjects of their investigations just ignore those subpoenas? That’s the question currently facing House Democrats, who have been stonewalled by the Trump administration at every turn so far. “We will not concede the Executive’s constitutional prerogatives or allow the Committee to jeopardize the individual privacy rights of current and former Executive Branch employees,” White House Counsel Pat Cipollone wrote to Oversight committee chairman Elijah Cummings last month, in response to a request for information regarding Jared Kushner’s controversial security clearance.

Cummings must be getting particularly tired of that line, as he’s gotten some version of it three times just this week. So far, the White House has blocked principal deputy assistant attorney general John Gore, former aide Carl Kline, and now Miller from testifying before Cummings’ panel. But it’s not clear what recourse Cummings and other House Democrats have in the face of the administration’s noncompliance. Democrats can hold members of the administration in contempt, but that’s more of a political move as opposed to an enforcement measure with any real teeth to it, as Rep. Jackie Speier told CNN on Wednesday. (“I’m afraid so,” she told Wolf Blitzer when asked if a contempt citation would be mostly symbolic.) Dems could also take the fight to the courts to get their subpoenas enforced, but that could take months or maybe even years, CNN noted Thursday—meaning these fights may no longer even matter by the time they’re resolved.

Some Democrats, including Speier, have called for court battles anyway. “I think the only subpoena to carry the clout necessary is through the courts,” she said. But even those who support it acknowledge that the move may not have any immediate impact. “I have no doubt we will prevail in court,” Rep. Ro Khanna told CNN. “The problem is this is a cynical strategy by the Trump administration to delay any investigation and attempt to run out the clock before 2020.” Indeed, 2020 looms large over all these investigations, as Democrats balance vows to hold the president accountable with a wariness to do anything that could hurt their chances to beat Trump at the ballot box. Party brass has already shown a calculated reluctance to impeaching Trump following the release of Robert Mueller’s damning Russia report, concerned that a divisive push to oust the president could give him campaign fodder. It’s possible Democrats will feel the same way about contentious court fights.

As such, it’s unclear what congressional oversight really means as it pertains to the Trump administration. It doesn’t seem so long ago that conservatives were whining about what they said was executive abuse by Barack Obama, whom they accused of sidestepping the legislative branch and not answering to Congress. But Trump is the out-of-control president they were griping about, enacting his most controversial measures through executive order and resisting efforts to rein him in. He has already made clear that he sees himself as above the law. In defying Congress, he’s making clear that he also sees himself as above the system of checks and balances that have been essential to American democracy. “We’re fighting all the subpoenas,” Trump told reporters outside the White House Wednesday. “These aren’t like impartial people. The Democrats are trying to win 2020.”

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