Bernie Sanders probably thinks he's out in front of the curve on this.

On Tuesday, the senator from Vermont published a polemical musing titled, "What should we do if the president is a liar."

The day before, in the wake of President Donald Trump's unsupported claim that former President Barack Obama had "wiretapped" his phones at Trump Tower, Sanders had sent out a series of tweets about Trump's propensity to brazenly lie.

"President Trump cannot continue to lie, lie, lie," he wrote in one tweet. "It diminishes the office of the president and our standing in the world."

Sanders' tweets, like so many of Trump's, are unusual. As a general rule, politicians do not call other politicians liars, no matter how much they disagree with their ideological opposites. Basic courtesy.

The Washington Post's Amber Phillips slapped at Sanders for abandoning this longtime Beltway etiquette. The headline of her article on Monday: "The sorry state of political discourse right now, in five Bernie Sanders tweets."

Such tweets only further poison the political atmosphere, making it even less likely Americans will find common ground on the important issues of the day, Phillips wrote of Sanders' social-media posts. Besides, she added, "[i]t's possible Trump believes the allegations he's making," in which case he wouldn't technically be lying.

"All of that," she concluded, "is why we in the media are careful not to call Trump a 'liar.'"

Sanders is having none of it. So he took to the website Medium to explain himself.

"What should a United States senator, or any citizen, do if the president is a liar?" he wrote, using italics as emphasis. "Does ignoring this reality benefit the American people? ... Or do we have an obligation to say that he is a liar to protect America's standing in the world and people's trust in our institutions?"

To be sure, Sanders isn't pointing out Trump's falsehoods only to ensure people's trust in our institutions. His problem with Trump goes far beyond the president's lack of regard for the truth. Here are a few excerpts from recent Facebook posts by the senator:

"Let's call it what it is. This [new travel] ban is a racist and anti-Islamic attempt to divide us up."

"This is how authoritarians operate. They scapegoat minorities and people with little power. Donald Trump's latest plan to publish crimes committed by undocumented immigrants is exactly that."

"Remember when Donald Trump told us that he was going to take on Wall Street and drain the swamp? Ha! What a joke."

Sanders has been a pioneer in various way during his public career. In a political system utterly dominated by the two major parties, he is, as he proudly points out, "the longest-serving independent in congressional history." Even more remarkable than that: he almost single-handedly rehabilitated the concepts of socialism in the U.S.

"Today, in America, for the first time in nearly a century, socialism is not a dirty word, or a shunned label, for many people," Politico wrote in March 2016, as Sanders' longshot campaign for the Democratic presidential primaries was gaining steam. Just six months earlier, USA Today had reported that more than 60 percent of Americans physically recoiled at the word. "The S-word -- socialism -- frightens a lot of Americans," the newspaper wrote, because it "still carries the associations with authoritarianism that it acquired during the Cold War."

But Sanders, who self-identifies as a "democratic socialist," says his reaction to Trump's falsehoods -- often unleashed by the president, it seems, to distract attention from controversies such as the investigation into his campaign team's contacts with the Russian government -- has nothing to do with ideology.

"It is easy to know how we respond to a president with whom we disagree on many, many issues," he wrote, listing disagreements with Trump over the Affordable Care Act, Wall Street regulations, environmental protections, "[a]nd on and on and on!" These differences of opinion, he insisted, are not why he calls the president a liar. He calls the president a liar because he can find no other appropriate word for Trump's behavior.

"[H]ow do we deal with a president who makes statements that reverberate around our country and the world that are not based on fact or evidence?" he asked.

Sanders undoubtedly feels out on his own on this subject. Amber Phillips is right that most mainstream news outlets still dance around directly calling the president a liar. In January, Wall Street Journal editor Gerard Baker said his newspaper would continue to avoid the word for the very reason Phillips listed this week. "I'd be careful about using the word 'lie,'" Baker said. "'Lie' implies much more than just saying something that's false. It implies a deliberate intent to mislead."

But Sanders shouldn't feel alone. In fact, there's a fellow senator who used the "L" word long before Sanders did -- in fact, the word helped launch his political career.

In 2003, when he was a not-very-famous comedy writer and performer, Al Franken wrote a book about conservative media personalities. The title: "Lies and the Lying Liars Who Tell Them." The book became a national bestseller.

Franken is now the junior senator from Minnesota. He's been in the news lately for his sharp questioning of U.S. Attorney General Jeff Sessions during Senate confirmation hearings, an exchange that ultimately led to Sessions recusing himself from the Justice Department's Russia-Trump investigation.

The "Lying Liars" author hasn't backed off his use of the forbidden word over the past 14 years. Unlike Sanders, who insisted in his Medium essay that he "strongly believe[s] in civil political discourse," Franken doesn't even limit his use of the "liar" tag to President Trump.

Said the Minnesota senator this week when asked about Sessions' response to his questioning:

"It's hard to come to any other conclusion than he just perjured himself."

-- Douglas Perry