It isn’t often the chief of staff of the U.S. Air Force, a person whose career has been to serve the country, publicly repudiates his commander in chief.

But that seems to be the only plausible reading of General Dave Goldfein’s pointed tweet on Wednesday afternoon: “I stand with my fellow service chiefs in saying we’re always stronger together — it’s who we are as #Airmen.”

I stand with my fellow service chiefs in saying we're always stronger together-it's who we are as #Airmen pic.twitter.com/9XxOry93nf — Gen. Dave Goldfein (@GenDaveGoldfein) August 16, 2017

In the context of the Charlottesville fallout, it’s hard to see another explanation.

Then again, it isn’t often a Republican president is forced to disband two business advisory councils because America’s CEOs are too embarrassed to associate with him.

And while we’re on the topic, it isn’t that often a president is publicly rebuked, and harshly, by legions of his own party, including the previous nominee:

No, not the same. One side is racist, bigoted, Nazi. The other opposes racism and bigotry. Morally different universes. — Mitt Romney (@MittRomney) August 16, 2017

If Donald Trump looks like a lame duck, walks like a lame duck, and quacks like a lame duck, is he still a viable chief executive of the U.S. government? Will he be able to get anything done? Will he have the clout to push through complex tax or infrastructure bills, or anything else meaningful?

Read: This is the lesson the stock market has learned from Charlottesville

And, importantly for MarketWatch readers: If he is now a political lame duck, what, if anything, is that going to mean for stocks, and your 401(k)?

The short answer, based on history, is ominous.

While the overall picture is mixed, in the handful of times in the past 100 years when the U.S. president has become too politically weakened to lead effectively, the Dow Jones Industrial DJIA, -0.28% as typically done badly, losing an average of 14% of its value in constant dollars, or when measured when adjusted for inflation. (I used inflation-adjusted data to eliminate distortions, particularly from the rampant price increases of the Jimmy Carter era.) That’s a pretty dismal result, given that overall the market’s general direction has been up.

More ominously still, in many cases the market suffered from the double whammy of presidential weakness and overvalued stocks — arguably the same situation that exists today.

Read: Trump’s pro-business agenda at risk after corporate America abandons president

I’ve ignored the usual periods between the election of a new president and the inauguration, only focusing on those unusual, extended periods when a sitting president is politically weakened.

On the positive side, the sample is small, and the list is inevitably subjective. You can quibble with my historical choices. In the case of two presidents — Ronald Reagan and Bill Clinton — they eventually shrugged off a scandal and reclaimed lost power.

But if there is a lesson to be drawn, it’s that when a president has lost so much political capital that he cannot lead the country or command its respect, the resulting power vacuum can be very dangerous — for investors as well as for everyone else.