Let's start with the tweets, which Sen. Bernie Sanders (I-Vt.) (and/or his staff) fired off late Monday morning:

Do me a quick favor. Hit pause on your temptation to either cheer or jeer what Sanders said and let this marinate: A prominent U.S. senator just described the president of the United States as a frequent and “shameless” liar, a claim that for reasons I'll explain is difficult to prove. What's more, what Sanders said about President Trump is one of a bazillion hefty criticisms that Democrats have lobbed and will lob at the president this week alone.

This isn't the first time a lawmaker has outright accused the president of lying. Remember former Rep. Joe Wilson (R-S.C.) yelling "you lie!" in the middle of President Obama's 2009 State of the Union address? "You lie!" was huge news then. In 2017, one side of the aisle is accusing the president straight-up of lying and that's just another day in politics.

This is the state of our political discourse right now. Political norms — like, don't accuse the president of the United States of lying without evidence, or don't accuse the former president of the United States of wiretapping your phones without evidence — have been eviscerated. There are no rules right now in politics about what you can/can't or should/shouldn't say.

Why should you care it exists at all? Well, the manifestation of such no-holds-barred rhetoric is often a lot of talk and not a lot of action.

Say you have a job, and say within that job you have a colleague, and say said colleague is, in your mind, a frequent and “shameless” liar. You're probably not going to want to give that colleague the time of day. The reverse is true, too: If you're asked to work with someone who just told the entire office that you're a liar, well, screw him, right?

That's why this ultra-tense Democrat-Trump relationship is more than just he-said, she-said politics. Because Senate Democrats can require 60 votes to approve any piece of legislation, they have leverage over Trump and congressional Republicans, who have only 52 members in the Senate. Trump needs at least a handful of Democrats to help him repeal and replace Obamacare. And carry out his plan to restructure the tax code. Or pass a $1 trillion infrastructure package.

Right now, the exact opposite is happening: Democrats are blocking Trump in historic ways, like stalling committee hearings for key Cabinet posts or threatening to filibuster his Supreme Court nominee.

Democrats will argue that it's Trump who has torn up all the rules and sent Democrats into resistance overdrive. Since becoming president, he's tweeted several eyebrow-raising claims about widespread voter fraud or potentially earth-shattering wiretapping of his phones. He's refused to provide any evidence to back up those claims, and he and the White House are demanding that Congress investigate.

This is not how presidents are supposed to act, say Democrats. And desperate times call for desperate measures to make sure the American people know what's up. That includes describing the president of the United States as lying “shamelessly.”

“When the president and his agenda pose a clear and present danger to a large number of Americans, Democrats need to consider doing everything they can to try to stop him,” Jim Manley, a former top aide to retired senator Harry M. Reid (D-Nev.), told me in January when we spoke about Democrats stalling Trump's Cabinet picks.

Whether what Trump is saying is a “clear and present danger” lies in the eye of the beholder. But plenty of nonpartisan observers would agree that the extraordinary claims Trump is making have no precedent in modern-day politics.

Trump is on the edge of being a conspiracy-theory president, writes The Fix's Chris Cillizza. The charge that President Barack Obama tapped Trump Tower, in particular, takes his presidency “onto a road with no centerlines or guardrails,” writes The Post's Karen Tumulty.

In that context, Democrats' mind-set seems to be: If Trump is not going to play by the rules, neither are we.

Here's the problem with using the “L” word in politics, though. To say someone's lying suggests that you know they don't believe what they're saying.

It's possible Trump believes the allegations he's making, which seem to have surfaced on a conservative news site one of his top aides used to manage. Several top House Republicans haven't brushed off the wiretapping claims, with Rep. Devin Nunes (R-Calif.), chair of the House Intelligence Committee, saying he'll look into it.

Some top Democrats, like Rep. Adam Schiff (D-Calif.), also aren't so sure Trump is intentionally distorting what he believes to be true.

All of that is why we in the media are careful not to call Trump a “liar.” But top Democrats like Sanders feel no such hesitation. In their mind, the president has become so unhinged that they have no choice but to accuse him of lying “shamelessly,” corrosive effects on political discourse be damned. If you're a Democrat, they were already up in smoke anyway.