What limits the amount of candidates running for presidential election?

In the last election, candidates like Duncan Hunter and Mike Gravel were very unpopular. I was surprised only 8 people ran for the Democratic ticket. Since a 3 year senator became president, now just about anyone is congress could run. What prevents like 30 people from running in each party?

Public Comments

1. peoples attention spans.
2. Only the strongest candidates have a chance at winning. When it comes down to it, the party will only support a candidate with a chance at winning. The rest will invest a great deal of money in a campaign and lose it all. So it's a money thing.
3. Nothing, but you have to be able to have the funds and a strong message to stand out fromt he pack
4. not sure
5. The only limit is ones ability to lie. Obama said he'd use tax payer support and then changed his mind so he could get foreign donations from our enemies. That was in keeping with his proclivity for associating with America hating terrorists; both foreign and domestic.
6. The swimsuit competition.
7. Nothing, no one.