It was a catchy political pitch and a chance to calm nerves about his dramatic and complicated plan to bring historic change to America’s health insurance system.

"If you like your health care plan, you can keep it," President Barack Obama said -- many times -- of his landmark new law.

But the promise was impossible to keep.

So this fall, as cancellation letters were going out to approximately 4 million Americans, the public realized Obama’s breezy assurances were wrong.

Boiling down the complicated health care law to a soundbite proved treacherous, even for its promoter-in-chief. Obama and his team made matters worse, suggesting they had been misunderstood all along. The stunning political uproar led to this: a rare presidential apology.

For all of these reasons, PolitiFact has named "If you like your health care plan, you can keep it," the Lie of the Year for 2013. Readers in a separate online poll overwhelmingly agreed with the choice. (PolitiFact first announced its selection on CNN's The Lead with Jake Tapper.)

For four of the past five years, PolitiFact’s Lie of the Year has revolved around the health care law, which has been subject to more erroneous attacks than any other piece of legislation PolitiFact has fact-checked.

Obama’s ideas on health care were first offered as general outlines then grew into specific legislation over the course of his presidency. Yet Obama never adjusted his rhetoric to give people a more accurate sense of the law’s real-world repercussions, even as fact-checkers flagged his statements as exaggerated at best.

Instead, he fought back against inaccurate attacks with his own oversimplifications, which he repeated even as it became clear his promise was too sweeping.

The debate about the health care law rages on, but friends and foes of Obamacare have found one slice of common ground: The president’s "you can keep it" claim has been a real hit to his credibility.

Why the cancellations happened

How did we get to this point?

The Affordable Care Act tried to allow existing health plans to continue under a complicated process called "grandfathering," which basically said insurance companies could keep selling plans if they followed certain rules.

The problem for insurers was that the Obamacare rules were strict. If the plans deviated even a little, they would lose their grandfathered status. In practice, that meant insurers canceled plans that didn’t meet new standards.

Obama’s team seemed to understand that likelihood. U.S. Health and Human Services Secretary Kathleen Sebelius announced the grandfathering rules in June 2010 and acknowledged that some plans would go away. Yet Obama repeated "if you like your health care plan, you can keep it" when seeking re-election last year.

In 2009 and again in 2012 , PolitiFact rated Obama’s statement Half True, which means the statement is partially correct and partially wrong. We noted that while the law took pains to leave some parts of the insurance market alone, people were not guaranteed to keep insurance through thick and thin. It was likely that some private insurers would continue to force people to switch plans, and that trend might even accelerate.

In the final months of 2013, several critical elements of the health care law were being enacted, and media attention was at its height. Healthcare.gov made its debut on Oct. 1. It didn’t take long for the media, the public and Obama’s own team to realize the website was a technological mess, freezing out customers and generally not working.

Also on Oct. 1, insurers started sending out cancellation letters for 2014.

No one knows exactly how many people got notices, because the health insurance market is largely private and highly fragmented. Analysts estimated the number at about 4 million (and potentially higher), out of a total insured population of about 262 million.

That was less than 2 percent, but there was no shortage of powerful anecdotes about canceled coverage.

One example: PBS Newshour interviewed a woman from Washington, D.C., who was a supporter of the health care law and found her policy canceled. New policies had significantly higher rates. She told Newshour that the only thing the new policy covered that her old one didn’t was maternity care and pediatric services. And she was 58.

"The chance of me having a child at this age is zero. So, you know, I ask the president, why do I have to pay an additional $5,000 a year for maternity coverage that I will never, ever need?" asked Deborah Persico.

The administration’s botched response

Initially, Obama and his team didn’t budge.

First, they tried to shift blame to insurers. "FACT: Nothing in #Obamacare forces people out of their health plans," said Valerie Jarrett, a top adviser to Obama, on Oct. 28.

PolitiFact rated her statement False . The restrictions on grandfathering were part of the law, and they were driving cancellations.

Then, they tried to change the subject. "It’s important to remember both before the ACA was ever even a gleam in anybody’s eye, let alone passed into law, that insurance companies were doing this all the time, especially in the individual market because it was lightly regulated and the incentives were so skewed," said White House Press Secretary Jay Carney.

But what really set everyone off was when Obama tried to rewrite his slogan, telling political supporters on Nov. 4, "Now, if you have or had one of these plans before the Affordable Care Act came into law, and you really liked that plan, what we said was you can keep it if it hasn’t changed since the law passed."

Pants on Fire ! PolitiFact counted 37 times when he’d included no caveats, such as a high-profile speech to the American Medical Association in 2009: "If you like your health care plan, you'll be able to keep your health care plan, period. No one will take it away, no matter what."

Even Obama’s staunchest allies cried foul.

On Nov. 6, columnist Clarence Page of the Chicago Tribune wrote that the public "was entitled to hear the unvarnished truth, not spin, from their president about what they were about to face. I don't feel good about calling out Obama's whopper, because I support most of his policies and programs. But in this instance, he would have to be delusional to think he was telling the truth."

The next day, Obama apologized during a lengthy interview with NBC News’ Chuck Todd.

"We weren’t as clear as we needed to be in terms of the changes that were taking place, and I want to do everything we can to make sure that people are finding themselves in a good position, a better position than they were before this law happened. And I am sorry that they are finding themselves in this situation based on assurances they got from me," he said.

Political fist-fight

The reaction from conservative talk shows was withering. On Nov. 11, Sean Hannity put Obama’s statements up there with President Richard Nixon’s "I am not a crook," and President Bill Clinton’s "I did not have sexual relations with that woman."

On the liberal network MSNBC, Joy-Ann Reid said the Obama administration’s intention was to fight off attacks like the ones that scuttled Clinton’s health proposals in the early 1990s.

"That’s why the administration boiled it down to that, if you like your health care, you can keep it. Big mistake, but it was a mistake that I think came a little bit out of the lesson" of the Clinton years, she said Nov. 12.

Two days later, House minority leader Nancy Pelosi defended Obama’s statement as accurate and blamed insurance companies. "Did I ever tell my constituents that, if they like their plan, they could keep it? I would have, if I'd ever met anybody who liked his or her plan, but that was not my experience," she said.

Obama offered an administrative fix that same day, allowing state insurance commissioners to extend current plans. But only some have chosen to do so.

In announcing the fix, Obama again conceded he had exaggerated. "There is no doubt that the way I put that forward unequivocally ended up not being accurate," he said. "It was not because of my intention not to deliver on that commitment and that promise. We put a grandfather clause into the law, but it was insufficient."

It is too soon to say what the lasting impact of "if you like your health care plan, you can keep it" will be.

The president’s favorability ratings have tumbled in recent weeks.

A Pew Research/USA Today poll conducted Dec. 3-8 found the percentage of people viewing Obama as "not trustworthy" has risen 15 points over the course of the year, from 30 percent to 45 percent.

Much depends on the law’s continuing implementation and other events during Obama’s final three years in office, said Larry Sabato, a political scientist who runs the Center for Politics at the University of Virginia.

Still, Obama has work to do to win back public trust, Sabato said.

"A whole series of presidents developed credibility gaps, because people didn’t trust what they were saying anymore. And that’s Obama’s real problem," he said. "Once you lose the trust of a substantial part of the American public, how do you get it back?"