Presidents who failed to sign any major piece of legislation in the first 100 days — especially presidents whose parties also controlled Congress — have sputtered in their first couple of years, too, said Brooklyn College history professor and presidential scholar Robert David Johnson. And that should be a major warning sign to President Trump, who on Friday tacitly acknowledged he has had a lackluster start.

“There really aren't a lot of examples — zero — of presidents who substantially struggled in the 100 days and then quickly rebounded,” Johnson said.

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A comparable parallel, Johnson said, was President Jimmy Carter, who struggled to whip up excitement within a Democratic-controlled Congress for pretty much anything, his first 100 days and beyond. He signed an energy bill a year and a half in, but it wasn't what he wanted, and that “pretty much sucked the oxygen out of his presidency.”

President Bill Clinton struggled with the perception he hadn't done much in his first year with Congress, and his party got slammed in the midterm elections. He lost control of Congress and never regained it.

The solid thread in a 100-days cautionary tales is Congress — specifically whether a president has built up a solid working relationship with Congress or has a total deficit of trust.

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Trump's relationship leans toward the latter. After the health-care implosion, Trump pointed his Twitter trigger finger at pretty much every faction of Congress — calling up The Post's Bob Costa and blaming Democrats, mentioning a show where the host demanded House Speaker Paul D. Ryan (R-Wis.) step down, starting a war with the House Freedom Caucus.

Basically, 100 days is a way to measure how well a new president has his feet under him. Trump still seems to be slipping with every step, said nonpartisan budget expert Stan Collender.

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“You would expect to have the president's government in place and a working relationship with the other branches by then. He's got problems with appointments and no relationship with Congress,” Collender said.

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And every day gets more difficult to work with Congress. That's because every day is a day closer to the 2018 midterm elections, when vulnerable Republicans may not want to be seen compromising with an unpopular president.

There are caveats to this 100-day measure: Like, for instance, we're dealing with a small subset of presidents who have been sworn in under similar circumstances (their own party also in control of Congress and responsible for big promises and expectations). Outside events could always occur that change the way Americans and skeptical members of Congress view Trump. (Right now he has historically low approval ratings.)

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You could also argue that a productive 100 days isn't indicative of a productive presidency: President Barack Obama signed an economic stimulus package in his first 100 days, went on to pass health-care reform, and then his party got wiped out in the midterms.

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But with regard to failure, a meh 100 days has historically been a pretty good measure of a meh first few years.

Speaking of failure, you could also argue Trump has set himself up for this. Up until the health-care implosion, he was pretty clear that the 100 days construct matters. More explicitly than any other president who came before him, he laid out exactly what he was going to get done in these first few months — renegotiate trade deals, military alliances, tax cuts, address Russian hacking.

None of that has come to fruition.