PHOENIX Defeated just two years ago as the Republican presidential candidate and with his bonafides as a true conservative again being challenged, John McCain finds himself in a struggle to get even his party's nomination for another term in the Senate.

Many conservatives and Tea Party activists are lining up behind Republican challenger and former talk radio host J.D. Hayworth, reflecting a rising tide of voter frustration with incumbent politicians. Only 40 percent of Arizonans have a favorable view of McCain's job performance.

Faced with his toughest re-election battle ever, McCain has moved to the right on several hot-button issues, like gays in the military and climate change, and has built a campaign war chest of more than $5 million. Former running mate Sarah Palin and newly elected Republican Sen. Scott Brown, both popular with conservatives, are pitching in.

Hayworth, who will officially launch his campaign Monday, began using his talk show on conservative radio station KFYI to drum up opposition to McCain.

"You have a consistent conservative challenger and an incumbent who calls himself a maverick but in fact is a moderate," Hayworth said, outlining what he views as the central choice for conservative GOP primary voters in August.

McCain is launching his own statewide tour, complete with visits next month from Palin and Brown, who already has recorded calls asking Republicans to support McCain.

The four-term senator and his allies also are taking aim at Hayworth. In December, they filed a complaint with the Federal Election Commission arguing that the talk show host was a de facto candidate and his radio station was providing a corporate gift by allowing him to campaign on the air. And they're attacking Hayworth's 12-year record as a congressman representing the eastern suburbs of Phoenix.

"When Arizona voters are reminded of Mr. Hayworth's long record in Washington of liberal spending and questionable ethics, he will be defeated again just as he was in 2006," said McCain campaign spokesman Brian Rogers.

Democrat Harry Mitchell defeated Hayworth four years ago, winning the GOP-dominated 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 combative partisan, highlighted by an editorial in the state's largest newspaper recommending "Mitchell over the bully."

Hayworth said he decided to quit the show and run for Congress in late January after holding "town-hall meetings five days a week" with his conservative listeners.

They are angry, Hayworth says, about McCain's history of teaming with Democrats on key issues. In the past decade McCain has worked with Sen. Russ Feingold of Wisconsin on campaign finance reform and with the late Sen. Ted Kennedy of Massachusetts on an immigration bill that would have created a pathway to citizenship for illegal immigrants.

Hayworth and his supporters are particularly troubled by McCain's immigration bill, which they call "amnesty" for illegal immigrants. McCain has since backed off his calls for comprehensive immigration reform, saying the government should focus first on securing the border before figuring out how to handle the millions of illegal immigrants already in the country. He says he opposes amnesty.

The four-term senator also backtracked on a pledge to support repealing the "don't ask, don't tell" policy banning gays from serving openly in the armed forces if top military officials did. McCain criticized senior military officials when they told Congress this month that they supported a repeal and would study how to pull it off.

That's not enough to erase a history of turning his back on conservatives, said Rob Haney, chairman of the Maricopa County Republican Committee covering the Phoenix area.

"We cannot depend on that man to be a conservative," Haney said.

McCain faces growing frustration at home. A poll last month by the Phoenix-based Behavior Research Center found his lowest approval rating since January 1994, when McCain was in the midst of the "Keating Five" scandal in which he and four other senators were accused of trying to intimidate regulators on behalf of a real-estate developer later convicted of fraud.

McCain's once-powerful support from independents is particularly lacking; just 38 percent approved of his performance. But among Republicans, McCain retains solid support; 51 percent approved and just 14 percent disapproved, according to the telephone poll conducted Jan. 7-22 with 629 registered voters statewide. It had a margin of error of five percentage points.

Arizona allows independents to vote in primaries. They could make the difference in a state where 30 percent of the electorate doesn't belong to a political party.

McCain is the clear front-runner, known for two presidential campaigns and almost three decades representing the state 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 his campaign is approaching $100,000 in contributions.

McCain is also no stranger to challenges from the right. He survived a barrage of criticism that he was too moderate and came back from the brink of political death to win the GOP nomination for president in 2008.

"He is one hard campaigner. He's never lost a race here in Arizona," said Doug Cole, an Arizona political consultant who worked for McCain in the 1980s and later for his presidential campaigns. "You can't count John McCain out."

Associated Press Writer Jacques Billeaud contributed.