WASHINGTON — It's Washington's new favorite Latin phrase: quid pro quo. On Capitol Hill, Democrats seeking to impeach the president are trying to prove that he engaged in a quid pro quo ("this for that") with U.S. aid to Ukraine. The president has insisted "there was no quid pro quo." But then last week, White House acting chief of staff Mick Mulvaney seemed to admit to a quid pro quo during his hapless news conference.

It seems as though the fate of the Trump presidency hangs on those three Latin words. Find proof of a quid pro quo and Trump is a goner.

But that's not exactly true. The United States engages in quid pro quos all the time when it comes to foreign assistance. Our aid is not charity; Americans expect to get something in return for it. We have leveraged U.S. assistance in exchange for a host of objectives: economic reform, democratic reform, better pursuit of corruption, access to strategically important areas and so on.

In 1978, Jimmy Carter agreed to provide Egypt with billions of dollars in foreign aid in exchange for making peace with Israel, as part of the Camp David Accords. That was a quid pro quo. In 2004, George W. Bush created the Millennium Challenge Account, which required countries to meet a host of eligibility requirements — free speech, free assembly, rule of law, property rights, transparency — before they could receive a grant of aid. That program was one big quid pro quo.

Even former vice president Joe Biden has admitted to a quid pro quo with Ukraine. He held up $1 billion in loan guarantees (a quid) to get them to fire a prosecutor who was not investigating corruption (a quo). This was perfectly legitimate, he says, and he may well be right.

So, there is nothing intrinsically wrong with quid pro quos. It just depends what the quo is.

But let's say Trump had in fact withheld U.S. aid to get Ukraine to cooperate with the Durham probe. There would be absolutely nothing wrong with that. The president, as the country's chief law enforcement officer, would be entirely within his rights to use aid as leverage to get Ukraine cooperate with an official Justice Department investigation. If it turns out that the quo was "investigate my 2020 opponent," as Ambassador William Taylor reportedly alleged during testimony on Capitol Hill Tuesday, then Trump is in big trouble; but if the quo is "cooperate with the Justice Department," that's perfectly fine.

So, while Democrats may be insisting that Trump delenda est — "Trump must be destroyed" — they will need something more than this quid pro quo to do it.

Follow Marc A. Thiessen on Twitter, @marcthiessen.