What should we do if the president is a liar?

Politicians lie all the time, it's what they do. Bernie cannot sit on a moral high horse when it comes to lying, just as no politician can. They phrase their lies as political truth. Which is kind of like obi-wan's "true from a certain point of view".

Trump doesn't do the typical political lie methodology which is why people freak over it. So let's call it something different like, shit-talking. That's what all the examples in the post can really be classified as, simple shit-talk and bluster. It's very common to see at all levels of human interaction, but it's uncommon in public politics because were used to "statesman". Instead practiced politicians tend to be more graceful in their rhetoric. Listen to what gets caught on open mics or in what is expected to be private meetings. Both sides do it.

A lie with actionable weight behind it, such as lying to cover up criminal violations, lying to start wars, and so fourth. Those type of things deserve something done. Shit-talking braggadocio should be ignored. Like all braggards, its for attention, whether positive or negative, getting attention is counted as success. If you want to do something effective deprive a braggart of attention.