In nearly two decades as sheriff in Arizona's most populous county, Joe Arpaio's carefully crafted reputation for being fearless and tough on crime helped him coast to victory time and again. But as he seeks a sixth term, the veteran lawman is running hard to keep his seat.

In TV campaign ads and event appearances, Sheriff Arpaio has been forced to tone down his brashness, fend off accusations of racial discrimination against Latinos, and counter recurring complaints over what critics say were lax investigations of sex crimes against children.

"It's been nasty," admits the Maricopa County sheriff, a Republican.

Long known in Arizona for his penchant to engage in unorthodox law enforcement tactics, such as outfitting inmates in pink underwear, Arpaio was thrust onto the national stage in recent years for his crusade against illegal immigration.

His sweeps in mostly Latino neighborhoods and his unwavering support for the state's divisive immigration law, SB 1070, won him admirers near and far. But it also brought lawsuits from Latinos and the Obama administration over various accusations that included racial profiling.

Arpaio's critics now are training the spotlight on his legal troubles in an effort to persuade voters that it's time for a new sheriff in town.

"This is the toughest race he's had so far," says David Berman, a senior research fellow at Arizona State University's Morrison Institute for Public Policy.

"I think there's a little bit of fatigue, he's been in office for 20 years, he's 80 years old," he adds. "He's been very controversial, and I think people are getting a little bit tired of him, frankly."

Although Arpaio still boasts bedrock support from his party, his foray earlier this year into the birther movement that questions President Obama's birthplace failed to win over GOP allies in the state legislature. Arpaio's probe, which rendered Obama's birth certificate fake, went nowhere.

Critics say Arpaio wasted precious resources on investigating a much-debunked theory outside of his jurisdiction that would've been better spent working cases in Maricopa County.

"Obviously, that upset a lot of people," says Carlos Sierra, co-chair of Citizens for Professional Law Enforcement, an anti-Arpaio group.

"He's just completely made that office a joke," adds Mr. Sierra, a Republican who has joined forces with Democrats and independents to target Arpaio.

The group recently launched a series of TV and radio ads that portray the sheriff in a negative light and encourage voters to cast a ballot for his main rival, Paul Penzone.

Mike Stauffer is in the race as an independent candidate, but it is Penzone, an articulate, retired police officer in his mid-40s, who has secured the endorsement of various Latino groups that Arpaio has alienated.

The sheriff won't give either of his opponents the time of day, let alone participate in a debate.

"Everybody knows around the world what I do, so there's nothing to debate about," he says.

That attitude riles his critics.

"This is the man with the largest budget in the county that will not debate because he's afraid, he's running from his record," says Randy Parraz of Citizens for a Better Arizona.

Energized by its successful push last year to recall former state Sen. Russell Pearce, the Republican architect of SB 1070, Mr. Parraz's group has since worked to unseat the sheriff.

But just as Parraz, politicos, activists, and Latino organizations turn up the heat, plenty of GOP faithful still back Arpaio. Polls by the two parties –there are no independent polling numbers for the sheriff’s race – show him still in the lead, and supporters have made his campaign the most well-funded, with more than $8 million in contributions, mostly from out of state.

The support Arpaio still enjoys – including that of a faded Hollywood star – was evident at an event last weekend in the east Phoenix suburb of Mesa.

"Joe Arpaio is one of the last American heroes who not only enforces the laws that he's supposed to enforce, but he's not afraid to stand up for what he believes in," actor Steven Seagal, who has participated in the sheriff's immigration raids, told an effusive crowd.

In an area that leans Republican, those attending the event were firmly planted in the sheriff's corner. Many held signs that read, "Fire Obama."

To Steve Han, the negative ads and allegations swirling around Arpaio are nothing but political slander in an election year. He doesn't believe a word.

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Arpaio is doing an all-around competent job and deserves to stay in office, Mr. Han adds.

"If things are going the way they're supposed to be going,” he says, “and the job is getting done by someone with experience, there's no reason to change."