(CNN) – John McCain said Wednesday he's compiled his initial list of potential running mates.
The presumptive Republican nominee said on his campaign bus that the list has approximately 20 names on it, but said it will take "weeks if not months" to narrow the list down.
"It's the same process that has been used by Democrats and Republicans," the Arizona senator said. "It's not an unusual thing. You put the list together and then you just could do a cursory kind of a look that I guess you could do on Google."
McCain also said he hopes to name his running mate before the party's convention in September, as has been the practice in several recent presidential elections.
"I'd love to do it earlier in the run than later, but it depends on the process," he said. "We just really haven't gotten far enough along in the whole thing to really be able to even predict what we're doing, seriously."
In 2004, John Kerry announced his selection of John Edwards in early July - more than six weeks before the Democratic convention.
But McCain said he did not want to rush the process, hoping to avoid what he said were mistakes past nominees had made in choosing a vice presidential candidate. Specifically, McCain singled out the first President Bush's selection of then-Sen. Dan Quayle who had "not been briefed and prepared for some of the questions."
"I'm a great friend of Dan Quayle's and I think he was a fine senator," McCain also said. "I just think that it was you know, a lot of people in retrospect would have thought maybe the process should have been...I just think you have to have a measured process, make sure that you have taken every, all the factors into consideration and then decide."
– CNN's Dana Bash and Alexander Mooney contributed to this report