An historical sounds right to folks who have heard it that way. A historical sounds right to the others. In all cases the other way is bound to sound wrong.
I have in my collection around 13 different stylebooks or grammar/usage books with no fewer than three different “rules” on this.
The easy one: If the H is pronounced, use A
If the H is not pronounced, use AN (AP stylebook from late '80s)
The next level: If the H is pronounced but the word has more than one syllable and the stress is on any but the first syllable, use AN. (This stylebook dates from the '60s; I think I got it for a college grammar class.)
(I think this is the one giving trouble.)
The third rule: If the word beginning in age derived from the French language then use AN.
This is from a '20s-era grammar book. I guess in those days everyone knew which words were derived from the French language. (Hotel, honor, history . . . wait, could this be the source of the problem?)
And many people do drop their aitches. Still others pronounce some of them as Ys, e.g.: Youston, Texas. I tend to use this the way I pronounce it, therefore: “A historical romance. A historic moment. A Houston-based company.”
It is basically the way it sounds, the same rule that gives us “A European country” and “an electrical outage.”