I'd take the train myself. If your flight is late, you can take the Leonardo Express from the airport to Termini as late as 12:06, arriving at 12:37. Then take the 12:50 EuroStar Italia train to Venezia Santa Lucia, arriving at 17:17. If your flight is on time, go directly to Termini and use your free time to buy food and wine for a picnic on the train. Buy your Rome-Venice ticket at the airport station when you buy your ticket for the Leonardo Express.
Granted, the Alitalia flight Jeff listed arrives at Marco Polo at 13:45. However, it will take some time before you can leave the airport and there is still the time and expense it will take to get to Venice. I wouldn't be surprised if taking the train will put you in the heart of the city before you could get there from the airport.