No, you do not have to know who will win.
At the time of writing this article, Polymarket has odds of Harris winning the election at ~47%.
Now head over to PredictIt and Harris is leading by 55%. Interestingly, the chance of Harris NOT becoming president is trading at 46%.
So Harris winning or not winning election is sitting roughly at a combined ~93%. Factoring transaction costs, that's 6-7% spread waiting for you to be picked up.
The Bet
Buy "No" on Harris on PredictIt at 46c and buy "Yes" on Harris on Polymarket at 47.4c, and you'll have a 6.6c lock on your money invested. That's a cool 7% return betting on a (almost) sure thing.
Wait! You said 30% in your clickbait title?
6-7% between now and January 20th is 18-45% annualized depending on when the market resolves. If the election is called a few days after Tuesday, November 5th, then your annualized return would be closer to 35%. If it gets dragged out all the way to inauguration day on January 20th, then annualized return drops to 20%. Read the footnotes of both contracts to understand how they will resolve.
Why are the two markets apart?
I can think of a few reasons.
Limits to Arbitrage - Polymarket is blockchain based so it is in theory accessible worldwide. PredictIt is New-Zealand based with fiat money and has many geographical restrictions and not even available in my home country of Canada.
Investment Limit - PredicitIt limits investment contracts per individual to $850 a contract. That's so little money, especially compared to the friction of funding an account that it's not worth the time for most people to arbitrage. Making several accounts is also not worth the hassle.
It is safe to assume that the audience of PredictIt is a regional subset of Polymarket and there is a lot more friction to use it. It clearly shows in the wider spreads and less liquidity.
Is this free money then?
Almost. There's never any free money. But in this case, almost.
In 2021, I traded in size an even more outrageous mispricing on Polymarket contracts for Inauguration day. In this case unfortunately you will have counterparty risk on two platforms, which is definitely something to consider. Crypto is risky, and you run several risks inherent to DeFi which you should be aware of. PredictIt is a regulated entity subject to the wrath of regulators and last major market similar to it, InTrade, went up in flames.
Election prediction markets are wild. Last time I bet substantial amounts was on Harris winning the candidacy at 90c, a full day after WSJ had reported that Harris' nomination was basically a done deal. That contract traded all the way to August 21st until it was official during the Democratic National Convention, a full 3-weeks after the outcome was effectively sealed.
This is not investment advice. All opinions are my own, and you should do your own due diligence, as always.