Not a new idea, but it's been a while.
1753: King Frederick of Prussia runs out of patience with his famous guest. Frederick had showered Voltaire with honors; Voltaire had responded... badly. His was a contrary nature. It ended with the great philosophe doing some jail time in Frankfurt. And by 1754 he'd been bounced from France too.
OTL he ended up in Ferney, just next to Switzerland. But before that, he'd briefly considered moving to Pennsylvania. Hey, Descartes went to Sweden, right?
Say he does.
Say further that he likes it well enough to stay a couple of years. If Voltaire arrives in 1754 or so, he'll probably be pleasantly surprised; Philadelphia will compare favorably to a provincial French town in terms of literacy and availability of books and journals. Of course, everything will still be several weeks behind Europe, and style will be sorely lacking, but it won't be nearly as dreadful as he might have feared.
Then -- just as he's getting ready to go back to Europe -- the Seven Years War kicks in. Poor Voltaire becomes an enemy alien! And while he'd be treated very well and given all possible liberty, he'd be stuck in America until 1763.
Some random thoughts:
1) Founding fathers. Most were barely out of knee pants during the 1750s. Washington was a young officer and Franklin had his printing business up and running, but Jefferson was a schoolboy and Hamilton a baby. Jonathan Trumbull, Sam Adams, and Roger Sherman would be grown men, but I don't think any of them spent much time in PA. So Voltaire probably wouldn't have direct contact with any of them except Franklin. He'd probably spend a lot of time with Franklin. (Though who knows how the relationship would evolve. Famous authors sometimes get along with their publishers, and then again sometimes not.)
2) Perhaps an even closer connection between the early US and France? Voltaire, maligned and persecuted in France for much of his life, instantly became a national icon and demigod the moment he died (don't ask me... it's a French thing, we wouldn't understand). In OTL the first generation of US political and intellectual leaders drew from a hodgepodge of British, French, and classical sources to form the new polity; "President" comes from the French, as does the idea of a Supreme Court.
Mmm, suppose some of the Roman elements in US political culture were replaced with French ones? A Chamber of Deputies or an Estates-General instead of a Senate or a Congress? Instead of "E Pluribus Unum", "Beaucoup en Une"?
3) Voltaire didn't much like slavery.
It got more complicated, of course, because he was a man of his time and he thought blacks were racially inferior. (Not genetically inferior, mind -- he seems to have believed that they would, over time, become more intelligent and civilized. But that day was many generations away.) Meanwhile, while disliking slavery generally, he was ambivalent about
slavery for the Negro; he opposed it in the abstract but didn't bother much about it in practice. He seems to have parsed his misgivings with an unusually weak line of reasoning: blacks practiced slavery on each other, so they somewhat deserved to be enslaved.
One wonders if this attitude would have survived closer contact with Atlantic slavery. Granted, Philadelphia was not South Carolina, but there were a fair number of blacks running around. It was also a hotbed of early abolitionism -- and the intellectual elites, the people Voltaire would be hanging around with, were disproportionately abolitionist. And while Voltaire could be a complete bastard sometimes, his natural tendency was towards sympathy with the oppressed. So I'm inclined to think he'd take a harder look at the Peculiar Institution, and come down more strongly against it.
It would be A Good Thing if the early Republic had another line of attack on slavery. The Founding Fathers, frankly, left a rather mixed intellectual legacy for Lincoln and other non-fanatic abolitionists to work with; too many of them were slaveholders, and too many others were at least trimming their sails on the issue. Having Voltaire as an honorary FF would make things easier, perhaps.
4) Okay, what about America's effect on Voltaire?
We probably still get an Essays on the Morals of Nations (1756) and a Candide (1759) though the details will surely vary. And Candide will not have its famous ending line about "we must cultivate our garden" -- that was autobiographical, as Voltaire, already well into his sixties, had settled down nicely in Ferney.
TTL when he gets back to France he'll be nearly 70, but -- assuming the New World hasn't hurt his health, and why should it? -- he'll still have about 15 years to live.
Any knock-on effects?
Doug M.