Senator John McCain appears set to romp home in Arizona's Republican primary, but only after tacking right and spending millions of dollars to counter a threat from a former conservative radio show host.

Voters also head to the polls Tuesday for primary elections in Florida, Vermont and Alaska and results are being closely watched as a litmus test of the anti-incumbent mood in the run up to mid-term elections in November.

Results such as Arizona may predict whether insurgent candidates, especially Republicans backed by staunchly anti-government Tea Party groups, will continue to make advances over those with more moderate views.

With the American economy still in the doldrums, US President Barack Obama is struggling for popular support and Republicans hope to tap in to those anti-establishment sentiments to win back majorities in Congress.

The Arizona primary features a crowded field of hopefuls jostling for positions in the US Senate, the House of Representatives, the state legislature and other offices.

The most important contest is between McCain, the Republican presidential candidate who lost out to Obama in the 2008 White House race, and challenger J.D. Hayworth, a former Congressman and conservative broadcaster.

Hayworth shocked many when early polls had him running close to McCain, but the four-time senator came racing back after shifting to the right on key issues such as illegal immigration and border security.

McCain has poured over 20 million dollars into the primary race, compared with his challenger's 2.5 million, in an increasingly bitter campaign in which he targeted Hayworth's alleged ties to a disgraced lobbyist.

His efforts have been rewarded, with recent opinion polls suggesting that the Vietnam war hero now has a 20-point lead over Hayworth, who billed himself as "The Consistent Conservative."

Hayworth has portrayed McCain as a liberal apologist and attacked him for his opposition to Republican tax cuts -- which Obama has promised to let expire -- and particularly for his shift on the hot-button issue of illegal immigration.

Officials in Arizona, which borders Mexico, argue the Obama administration has failed to secure the borders, and they are overwhelmed by illegal immigrants.

Arizona Governor Jan Brewer signed a draconian new bill into law in April, but a judge last month stripped it of key powers allowing police to spot-check the immigration status of all suspects.

Hayworth has received the support of Tea Party conservatives with his calls for tighter border security against "illegal and criminal aliens."

The Tea Party movement, which sprung up in 2009 as a grass roots revolt against Obama's tax, economic and health reform policies, has electrified the Republican Party base.

Taking its name from a revolt against British rule in colonial Boston in 1773, the group has emerged as a powerful force in nominating Republicans for November's mid-term legislative and gubernatorial elections.

Tea Party candidates have already won important Senate primary victories in Utah, Colorado, Nevada, and in other states, over more mainstream Republicans.

Brewer, her campaign for re-election buoyed by strong approval of her support of the controversial immigration law, faces a single challenger after her other opponents dropped out of the race.

After vowing to take the immigration fight to the US Supreme Court, she is expected to roll to an easy primary victory and is now favored to win re-election in November.

In Florida, a fierce fight had been expected for the Republican Senate candidacy, pitting outgoing popular Republican Governor Charlie Crist against Tea Party candidate Marc Rubio.

But in April, Crist announced he would run as an independent, throwing the drama forward to the final vote in November.

In Alaska, Republican Senator Lisa Murkowski is hoping her long-time ties to legendary Alaska politician Ted Stevens, who was killed in a plane crash earlier this month, will help her best another Tea Party candidate, Joe Miller.

Miller is a veteran backed by the state's former governor Sarah Palin, but Murkowski has emphasized that her seniority will help bring Alaska funds, and so far she is leading polls.