Other posters have already come up with Huey Long, Charles Coughlin, Joe McCarthy, Lyndon LaRouche...

No American Presidential candidate before Trump has been so widely popular whilst also having a cult following of people who basically believe in an entirely different reality whilst also being so brash and brazen about it.

There have been demagogues before, with cultish followings, but they've not been anywhere near as popular as Trump.

To attempt to add a few:

Technically, Joseph Smith, founder of Mormonism, attempted to run for President back when Mormons were basically what we would now call a domestic terrorist group, and when most non Mormons viewed them as a dangerous cult.

He was assassinated by a mob, who stormed the jail he was in whilst awaiting trial for treason and other charges, before the election took place.

Also, you might be able to consider the fairly brief existence of the Anti-Masonic party at least somewhat akin to the living in a totally different reality attribute of MAGA people.

Basically, following the inciting incident of the Morgan Affair, where a William Morgan was apparently planning to publish a book outlining the evils of a Freemason conspiracy to control government and business in the US, but he was jailed, a bit of a circus trial ensued, and then he disappeared.

The Anti Masonic party was the US's first third-party and basically it was built off of what we'd now call conspiracy theories stemming from the Morgan Affair, and called for Masons to renounce their fraternity or to be uprooted from positions of prominence.

Much like the modern MAGA movement, it was full of highly religious conspiracy theorists, but it didn't really coalesce into also being a cult of personality around any of their more prominent members the way such reverence exists for Trump.