Hayworth began his campaign Monday with a rally in Phoenix and is set to go on a three-day tour across Arizona.
"Arizona needs strong, reliable, conservative leadership ... to stand up to the Democrats' leftist agenda and offer the conservative solutions we need to get our country moving again," Hayworth said, as he positions himself as the race's reliable conservative.
Conservatives in Arizona have long been skeptical of McCain, in part, for working with Democrats on such issues as campaign finance reform and immigration.
Even "Joe the Plumber" criticized McCain Monday for "trying to use" him.
McCain has however evaded political threats from the right and lately has staked out solidly conservative positions.
McCain took a swipe at his challenger at his own campaign event in nearby Tempe around the same time as Hayworth's, reminding listeners of questions that were raised about the former congressman's dealings with disgraced lobbyist Jack Abramoff.
Referring to Hayworth's alignment with the Tea Party movement, McCain praised it as a powerful initiative with strong supporters who "fought against these earmark projects that Mr. Hayworth so proudly proclaimed that he would get for his congressional district.
"Unfortunately that practice led to corruption, people like Abramoff and other members of Congress who went to jail," McCain said.
More than two dozen Arizona mayors vowed to support him.
Hayworth received his own endorsement. A co-founder of the anti-illegal immigration Minuteman group who already had been running against McCain dropped out, and endorsed Hayworth. Chris Simcox said the name on the ballot matters less than the message of the candidate.
As evidence that conservatives can't trust the four-term senator, Hayworth pointed to a series of McCain flip-flops that he calls "campaign-year conversions" on issues including gays in the military, climate change, campaign finance and immigration.
Hayworth has also lined up big-name conservative backers including Maricopa County Sheriff Joe Arpaio, known for his tough policies targeting illegal immigration, and conservative commentator Pat Buchanan.
"There are two John McCains," he said. "The one who campaigns like a conservative and the one who legislates like a liberal."
McCain has aligned his own list of prominent conservative backers, including his former running mate Sarah Palin and recently elected Massachusetts Senator Scott Brown, both of whom will campaign for McCain in Arizona next month. He also has the support of Arizona's Republican congressional delegation, including Representative Trent Franks, who endorsed one of McCain's opponents in the 2008 presidential primary.
Americans for Tax Reform President Grover Norquist has also backed McCain, saying he's been a consistent voice opposing government spending and has never voted for a tax increase. McCain has, however, opposed tax cuts, saying taxes should only be cut in conjunction with spending.
McCain is the clear front-runner, well-known after two presidential campaigns and almost three decades representing Arizona in Congress. He also has more than $5 million in the bank, not including the $2.5 million he's already spent, according to his latest campaign finance report. Hayworth said last week his campaign is approaching $100,000 in contributions.
A former television sportscaster, Hayworth was among a wave of Republicans elected to the U.S. House in 1994. He spent the next 12 years representing his district covering part of the eastern suburbs of Phoenix and, for a time, American Indian reservations.
Democrat Harry Mitchell defeated Hayworth in 2006, winning the Republican-leaning district amid a rough national climate for Republicans and questions about Hayworth's dealings with disgraced lobbyist Jack Abramoff.
Hayworth ran a conservative campaign emphasizing his opposition to illegal immigration. But he was dogged by a reputation for being an angry and bombastic partisan, highlighted by a scathing editorial in the state's largest newspaper recommending voters choose "Mitchell over the bully."
Cleora Fritz, 78, said she's voted for McCain since his first Senate run in 1986 but will be supporting Hayworth because "he's a true conservative and he wants to protect our borders."
"We live in this state, and we know the consequences for the entire nation of letting illegals cross our border," Fritz said.