President Donald Trump arrives at the Roosevelt Room of the White House in Washington on Friday. Credit: Evan Vucci | AP

White House Chief of Staff Mick Mulvaney admitted Thursday that President Donald Trump conditioned U.S. aid to Ukraine on a politically motivated investigation into the origins of the allegation that Russia hacked the Democratic National Committee. Then he attempted to reverse himself.

Regardless of whether the reversal succeeds, Mulvaney’s initial admission signals what looks to become a new direction for defending the president: Admit there was a quid pro quo with Ukraine, but claim the president has the constitutional authority to do such deals with foreign countries.

This is a defense that Democrats in charge of the impeachment inquiry should confront head on — sooner rather than later. It’s not enough simply to state that Trump abused his power when he offered Ukraine aid or a White House visit in return for investigating what Trump wanted investigated. The Democrats are going to have to explain why pressuring a foreign power to investigate political rivals subverts the very nature of republican self-government.

Start by taking the new Trump argument on its own terms. It’s a version of the idea that William Barr put forth in a memo to the Trump administration before he became attorney general last winter.

Then, Barr was addressing the allegation that Trump obstructed justice when he fired FBI Director James Comey to serve his own interest in avoiding investigation. Barr claimed that Trump could not have obstructed justice, because the Constitution grants him the authority to fire subordinates regardless of his motive, however self-interested it might be.

The new version of this argument, glimpsed in Mulvaney’s admission, is that the president’s constitutional authority to conduct foreign policy allows him to withhold aid to any country for whatever reason he likes. As Mulvaney explained it, withholding aid is something the executive branch does all the time; it’s just part of the president’s job. By extension, a quid pro quo is business as usual, not an abuse of power.

Of course, presidents normally withhold and grant aid to other countries in order to advance U.S. foreign policy interests, not to help get themselves re-elected. And it’s normally assumed that motive and effect are what distinguish ordinary foreign policy leverage from abuse of power. When the president advances only his own partisan interests, he abuses his power.

The inherent-power defense refutes that assumption by asserting that motive and effect don’t matter.

To counteract this claim, the Democrats will need to make explicit to the public why a quid pro quo to advance Trump’s partisan self-interest is worthy of impeachment. They must make the case that republican self-government — the goal of our democratic Constitution — depends on having a fair fight between candidates for election. When a president urges a foreign state to find evidence against his opponent, he violates this fairness. He takes advantage of his official power to manipulate the system — power his opponent lacks. This exploits the advantage of incumbency to undermine the possibility of a fair election.

No quid pro quo is necessary for this abuse of power to take place. In fact, Trump’s public appeal to China to investigate Joe and Hunter Biden is just as much an abuse of power as the Ukraine quid pro quo is. It is the president using his office to gain an electoral advantage not available to his rivals.

It follows that it’s no excuse to say the president is using a power granted by the Constitution. To the contrary: The abuse of power depends on possessing that power in the first place. To deploy presidential authority to gain partisan advantage is to abuse that authority.

Trump’s supporters are already struggling mightily to defend him. Contradictions in their efforts are to be expected, as in Mulvaney’s pullback from his initial statement. When the facts are clear and damning, however, Trump can be expected to attempt a constitutional defense. The Democrats should block this argument before it gains traction by reminding the public that a republic can’t survive if the leader can manipulate his powers to get himself re-elected.

Noah Feldman is a Bloomberg Opinion columnist. He is a professor of law at Harvard University.