Presidents boast a dismal track record on predicting when things will become unglued politically.

George W. Bush never imagined that his choice for director of the Federal Emergency Management Agency and a throwaway line of encouragement in the aftermath of Hurricane Katrina would haunt his entire second term.

But 10 days after Bush gushed to Michael Brown, “Brownie, you’re doing a heck of a job,” the FEMA director resigned amid charges of ineptitude. Bush’s handling of Katrina caused the president’s popularity — even among Republicans — to plummet to near record lows.

Donald Trump may never think about the consequences of his actions — beyond a flickering anticipation of how they will be portrayed on “Fox and Friends.” But even if the president had been in an uncharacteristically reflective mood, Trump probably would not have anticipated the uproar over last week’s impulsive tariff decision.

Trump’s determination to use a president’s tariff powers had long been hidden in plain sight like Edgar Allan Poe’s “The Purloined Letter.” As my colleague Patricia Murphy has pointed out, Trump’s full-throated opposition to free trade was highlighted at almost every campaign rally.

Watch: Ryan Says There’s a ‘Smarter’ Way to Go on Tariffs

Congressional Republicans may have been guilty of taking Trump seriously but not literally — to use journalist Salena Zito’s line from the 2016 campaign. With free traders at Treasury and in key White House posts from Chief of Staff John Kelly on down, there was an establishment consensus that Trump would not be allowed to unleash his inner protectionist.

But petulant over the impending resignation of the unflappably loyal Hope Hicks and the downgrading of his son-in-law Jared Kushner’s security clearance, Trump launched a global trade war like Rumpelstiltskin stamping his feet in anger.

In this grim reality tale, Trump’s decision was announced at a meeting with steel and aluminum executives with less staff preparation than a menu change in the White House Mess.

As Stephanie Ruhle and Peter Alexander of NBC News put it in chronicling the background of Trump’s fateful decision, “There were no prepared, approved remarks for the president to give at the planned meeting, there was no diplomatic strategy for how to alert foreign trade partners, there was no legislative strategy in place for informing Congress and no agreed upon communications plan.”

The results were cringeworthy moments like wealthy Commerce Secretary Wilbur Ross holding up a can of Campbell’s chicken noodle soup to demonstrate that its cost would go up by less than a penny under the Trump tariffs. What marred this man-of-the-people demonstration was the yacht basin visible in the background.

Sticking their necks out

Trump, who has turned congressional Republicans into a personal pep squad, did not expect dissent from conservatives, who suddenly remembered that they had a few principles left.

Paul Ryan — a disciple of free traders like Jack Kemp — somehow mustered the gumption to say through a spokeswoman on Monday, “We are extremely worried about the consequences of a trade war and are urging the White House to not advance with this plan.”

All through the Trump years, Ryan has taken his gospel from “Casablanca” —  most notably, Humphrey Bogart’s line: “I stick my neck out for nobody.”

That was why Ryan’s dissent, albeit delivered secondhand, seemed a sign of a significant break. By Tuesday, of course, Ryan had once again misplaced his copy of “Profiles in Courage.”

Speaking to reporters, the House speaker was at his wishy-washy best as he said, “What we’re encouraging the administration to do is … to be more surgical in its approach so that we can go after the true abusers without creating any kind of unintended consequences or collateral damage.”

True, Ryan and other prominent congressional Republicans may have deliberately tempered their public rhetoric in the hopes of convincing Trump to retreat before he formally imposes the tariffs.

It would be a typical Trump move, like, say, the always elusive Infrastructure Week. Trump would make a grand gesture in public on trade to reassure his ardent supporters and then deliberately (or ineptly) fail to follow through and hope that nobody at Fox News would notice.

Remember, this is a president who tweeted, “Trade wars are good, and easy to win.”

Much of life is a reality TV show for Trump, with a dollop of primitive negotiating strategy thrown in. But protectionism may be one of Trump’s rare core beliefs — like his conviction that Ivanka and her husband Jared are exemplary public servants.

The way of the lemming

For congressional Republicans, the proposed tariffs and the risk of an unending cycle of retaliations and reactions should represent a moment of truth.

If GOP congressional leaders abandon free trade as the latest sign of their fealty to President for Life Donald Trump, then Paul Ryan and company should never again be allowed to use the word “principle” without listeners bursting into giggles.

Politically, a supposedly easy-to-win trade war runs the risk of transforming the 2018 elections from a GOP retreat into a rout. Every time Walmart has to raise prices because of Trumpian tariffs, it would be in the company’s best interest to honestly tell customers why imports from China suddenly are more expensive.

Ryan recognized these risks when he said in Monday’s statement, “The new tax reform law has boosted the economy, and we certainly don’t want to jeopardize those gains.”

A Trump trade war — launched by whim — has the capacity to dominate the 2018 campaign. If congressional Republicans willingly follow Trump off this cliff, then the GOP’s symbol should become the lemming.