Those feelings may complicate his decision whether to seek the presidential nomination for 2008, said McCain, R-Ariz.
"If I run, and we'll decide that early next year, there's a lot of work to do," McCain said as he began a two-day visit to Iowa, which traditionally holds leadoff caucuses in January of presidential election years.
"Here in Iowa there are parts of the party where there's still lingering resentment over the bitterness of the 2000 race," he said.
In 2000, McCain skipped the Iowa caucuses and opened his campaign with the New Hampshire primary. He beat Bush there, but the Texas governor overtook him in later primaries.
As he considers a possible bid for the GOP nomination in 2008, McCain has visited Iowa often. Most polls show him better known that other possible Republican candidates.
"Since we haven't decided whether or not to run, we haven't decided whether to compete here, but I think you could make the argument that it's very different than 2000," he said. "In 2000, I was the outsider and, you know, we could afford to pass up on Iowa."
This time, he said, "I think the nomination would be up for grabs, I really do."
McCain has worked to deflect criticism of his opposition to subsidies for ethanol, an important issue in corn-growing Iowa.
"My position on ethanol is support for ethanol when oil went over $40 a barrel," he said. He said soaring gasoline prices make ethanol competitive in the marketplace.
"I do not support subsidies, but I support ethanol and I think it is a vital alternative energy source, not only because of our dependence on foreign oil but because of its greenhouse reduction effects," he said.
McCain's visit included a stop Tuesday in Grinnell, where he campaigned for state Rep. Danny Carroll.