US President Donald Trump's approval rating will continue to drop but there's little Republicans in Congress can do.

OPINION: US President Donald Trump has been widely and appropriately condemned for reacting to the horror in Charlottesville, Virginia, with remarks about "both sides," and for comparing Confederate generals to George Washington and Thomas Jefferson, and for reacting to white supremacist terrorism by saying "there is another side."

Sure, I've been critical of the left's "antifa" tactics to counter fascism. (Among other things, they enabled the white nationalists to cry victimhood in Charlottesville.)

But the time to critique the left fringe is not after someone with vile racist views has just murdered a protester. That moment is the appropriate time to condemn white supremacist ideology, denounce the murderer, and express the hope that the nation can learn to live together better as we mourn our dead.

SHANNON STAPLETON/REUTERS A confederate flag is seen on a light pole with the name of US president Donald Trump in the Outer Banks of North Carolina.

This should go without saying, but apparently it doesn't, so there, I said it.

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And a lot of Republicans expressed that appropriate sentiment.

House Speaker Paul Ryan, a Republican from Wisconsin, for example, tweeted: "We must be clear. White supremacy is repulsive. This bigotry is counter to all this country stands for. There can be no moral ambiguity." And there you have it: The leader of the House has taken a stand against Trump.

A lot of liberals want to know how Ryan was going to stop Trump from saying things like this.

Republicans have learned that they can't. The party has, after all, tried to stop Trump from Trumping. It has tried surrounding him with party stalwarts who could advise him not to say things like this. It has tried condemning his most offensive and un-American utterances. It has tried making fun of his hands. None of it seems to have made much impression on Trump.

Our system of government leaves Congress with few levers to pull once sweet reason and appeals to the presidential self-interest have failed. If Trump wants to see how much lower he can push his approval ratings, there is little his party can do to stop him.

They can impeach him, of course.

But what would you put in the articles of impeachment? "In a press conference on August the 15th, President Donald J Trump said the wrong thing"? His response was hideous, but I have a tough time making it fit any reasonable definition of "high crimes and misdemeanours."

While the framers had a pretty good idea of what constituted such an offence, they don't seem to have given anyone besides Congress the power to decide exactly what circumstances constituted a high crime and misdemeanour.

As far as I can tell, the Congress of the United States can impeach a president because they don't like the way he combs his hair, and all the rest of us could do is write indignant letters to our local representative.

But in practice, Congress is limited by both political expediency and a due respect for the delicacy of our political institutions.

If Republicans impeach Trump over a press conference, indignant voters will turn the nation blue at the next election. But even if the GOP were willing to make that sacrifice for America, it's not clear that America would benefit, even if you think Trump is the worst president in the history of our nation.

Washington elites stepping in to remove Trump because they don't like what he says would validate exactly the complaints that led to his election in the first place.

Voters knew that he said things like this and voted for him anyway. Removing him for such a reason would call the democratic legitimacy of the government into question.

The damage to our institutions might well be even deeper than the damage that Trump is doing with his incompetent administration and polarising rhetoric. Especially if this becomes a precedent for future presidencies under perpetual threat that a restive Congress will decide to give the vice president a try.

At the point when, say, two-thirds of the country wants him removed, then Congress has the ability, and probably the duty, to do so. And given that the public is now about equally split on the question, we may get there.

But at the moment, Paul Ryan, like the rest of us, can do very little except watch in horror, and try to stand up for the good when our president won't.

- Megan McArdle is a Bloomberg View columnist. She wrote for the Daily Beast, Newsweek, the Atlantic and the Economist and founded the blog Asymmetrical Information. She is the author of The Up Side of Down: Why Failing Well Is the Key to Success.