The indictment of former New York City Police Commissioner Bernard Kerik on a long list of federal charges Friday could turn into a huge weapon for Republican presidential contenders trying to topple frontrunning candidate Rudy Giuliani.

Kerik is much more than just a former associate of Giuliani. He rose from being the former New York mayor's driver to become head of the city's corrections department, then assumed the prestigious post of top cop in America's largest city.

Giuliani so far has maintained a strong lead in the polls despite talk of his two divorces and moderate positions on gun control and abortion. But now he faces a new reality: a close friend and business partner potentially facing prison time for a litany of federal offenses.

For Giuliani, Friday's indictments raise the specter of constant reminders of an unsavory association that could haunt him for the remainder of the campaign.

Law enforcement officials on Friday described Kerik's tale as one of a public servant selling his office to the streets. He was charged in the 29-page indictment with multiple counts of tax fraud, conspiracy and making false statements. Kerik pleaded not guilty.

The Kerik indictment could raise difficult questions for Giuliani, a former U.S. attorney whose reputation hinges on his crime-fighting, mob-busting record, as well as his work alongside Kerik during the Sept. 11 attacks.

Raw Data: Indictment (pdf)

"These ... issues are very sad and very serious and have to be dealt with in the federal justice system that I have enormous respect for," Giuliani said Friday in Nevada, adding that he doesn't think the charges will dog him during the heat of the primaries early next year.

But even before the indictment was announced, GOP rival John McCain cited Giuliani's relationship with Kerik as a reason to doubt his opponent's judgment.

The Arizona senator was on the trail in New Hampshire Friday with former Homeland Security Secretary Tom Ridge. Kerik would have succeeded Ridge in 2004, had he not been forced to withdraw his nomination once news of the allegations surfaced. It was Giuliani who endorsed Kerik for the job.

McCain on Friday faulted Kerik not for the corruption charges, but for doing an irresponsible job of training police in Iraq.

"He stayed two months and one day left, just up and left," he said. "That's why I never would've supported him to be the head of homeland security, because of his irresponsible act when he was over in Baghdad to try and help train the police."

McCain then moved on to call Giuliani too inexperienced to be president.

Even Ridge jabbed at the former New York mayor, likening Giuliani's support of Kerik to "urban city patronage."

"It was clear the mayor and I had a different view what the department does and the kind of leadership it needed," Ridge told reporters. "His judgment would have been different than mine."

Giuliani spokeswoman Katie Levinson called McCain's comments a sign of "pure desperation."

Republican candidate Mitt Romney's campaign also released a statement Friday morning highlighting the former Massachusetts governor's ethical record. In it was a 2-month-old quote from Romney in which he said, "We can't have ethical standards that are a punch line for Jay Leno."

Romney, speaking during a campaign stop in Atkinson, N.H., said the Kerik indictment was "sad and disappointing," but would not comment on the potential implications for Giuliani's campaign.

In the hours before the indictment late Thursday, Giuliani made no reference to Kerik while speaking to donors in Las Vegas. He stuck to highlighting his record as New York mayor and billing himself as a realist on foreign policy.

At a stop earlier in the day in Iowa, Giuliani addressed the matter with the consistent message he's used for months and that he reiterated later Friday in Nevada, expressing contrition for his mistake and loyalty at the same time for his former business partner.

"I have pointed out that I think I made a mistake of not checking him out more carefully. But when you balance that mistake against all of the successes that we had, and the reality that you make some mistakes and you make some correct decisions, I think the overwhelming record is a record of great success," Giuliani said Thursday morning during a campaign stop in Dubuque, Iowa.

Giuliani appeared to find solace in the confirmation of another friend, Michael Mukasey, whom the U.S. Senate confirmed Thursday night to be the next attorney general.

Giuliani underscored his confidence in Mukasey in a statement, praising him for his knowledge of the federal justice system and his "distinguished career" as an assistant U.S. attorney and judge.

But as the candidate girds for criticism on Kerik, some rivals might choose to keep quiet.

Fred Thompson, mum on the matter so far, recently found himself defending an associate. Phil Martin, a close ally to Thompson and businessman who lent Thompson his plane, resigned his post Monday as a top campaign bundler, a day after his drug-dealing past was revealed in the press.

"I really can't comment (on Kerik)," Thompson said Friday after announcing his Social Security plan in Washington, D.C. "I heard about it awhile ago, and I really don't know anything about the facts of that case."

FOX News' Carl Cameron, Shushannah Walshe, Serafin Gomez and Cristina Corbin and The Associated Press contributed to this report.