All politicians lie, it's just a matter of how much. They can get away with telling some big ones because most people interested in politics, especially now with the US presidential primary's going on, are not fact-checkers, preferring to take at face value whatever the media, or worse yet, SOCIAL media, tells them. Even if there was such a thing as a “Truth-O-Meter” veteran politicians would probably not score badly though because they are veteran liars and are apt choose their words with great care. Of course, it's not possible to check absolutely everything a candidate says, but it's a good idea to ask what's the source and is something being taken out of context.
The waters are muddied by the fact that the news media are often nothing more than a public relations outlet for certain candidates and are willing to mislead, slant, and falsely report on national and international events. This is propaganda, not news. Add to that the fact that many political ads and debates do not focus on issues, but on mudslinging and calling their opponents everything they can think of from outright liars to racists.
It's long been known that if you tell a lie big enough, loud enough, often enough, and do it with authority, people will believe it. Just read the story about the time Upton Sinclair ran for governor of California! Smithsonian article.
About the only decent political election the U.S. had was the one that was won by George Washington, but after that one, candidates emphasized the shortcomings, real or made up, of their opponent and the campaigns, to use one writer's apt expression, resembled something between a carnival and a bar fight.
In the 1796 the Boston Independent Chronicle alleged that during the Revolution John Adams had publicly supported Washington but secretly attempted to have him cashiered out of the Army. There was such an attempt, but it was Sam Adams, a second cousin, who was behind it. Adams’s opponent was the highly revered Thomas Jefferson; at least he's highly revered today. But at election time he was portrayed as being the son of a half-breed Indian and a mulatto father and it was claimed that if elected President, there would be a civil war and a national orgy of rape, incest, and adultery.
Ol' Hickory, Andrew Jackson, was portrayed as a bloodthirsty wild man, a brawler, the son of a prostitute and a black man and it was said his older brother had been sold as a slave and Jackson put to death soldiers who had offended him. Throw in the fact that Jackson and his wife were, technically speaking, adulterers and you have to wonder who would ever have voted for him. What happened was Jackson's wife, Rachael, had married him before her first husband divorced her. Opponents screamed that an adulteress and her paramour husband should never be President “of this free and Christian land.” Rachael died of a heart attack before they could move into the White House. It was said of Jackson he was a gambler, a cock fighter, a slave trader and the husband of a really fat wife.
Back in 1839 Martin Van Buren was accused of being too close to the Pope. Actually he had official correspondence, as most all Presidents have, with the Vatican as part of his job as Secretary of State under Andrew Jackson. Opponents spread the word that there was some kind of Popish plot going on.
During the James K. Polk versus Henry Clay race of 1844 the Ithaca, New York, Chronicle ran a fake report that someone named Baron Roorback (a fake name) had personally witnessed Polk purchase 43 slaves. Opponents accused Clay of gambling, dueling, womanizing and swearing.
Abe Lincoln was called about every name you can think of: ape, ghoul and traitor. During the Civil War his wife was accused of collaborating with Confederates. That was considered so serious a charge that on his own Lincoln appeared before a Senate committee that was secretly examining the allegations and swore that Mary Todd Lincoln was innocent.
In 1884 the Buffalo Evening Telegraph accused Grover Cleveland of fathering an illegitimate son in 1874 in Buffalo. That one may have been true. Cleveland, a bachelor, had dated the kid's mother, but so had a lot other men, so there was no way of knowing for sure who's son he was. It was kind of suspicious though that Cleveland had provided for the kid. The result was that his opponents took up the slogan: “Ma! Ma! Where’s my pa? Gone to the White House, ha! ha! ha!”
Cleveland’s opponent, James G. Blaine, was not exactly a saint either. Blaine was involved in a business scandal when a railroad had permitted him to sell bonds for a generous commission in return for a land grant. Blaine, in a cover-up attempt, had ordered the evidence burned, but he couldn't get away from the damage. “Blaine, Blaine, James G. Blaine, the continental liar from the State of Maine” became the slogan used by his opponents.
I could go on, but what's the point? It would be funny if it wasn't so serious.