WASHINGTON — When Democrats this spring sized up Al Franken’s bid to win a Senate seat in his native Minnesota, they saw plenty of promising signs: an engaging and famously funny candidate familiar to voters, a stockpile of campaign cash, and a vulnerable incumbent Republican.

Less than three months before Election Day, however, the Republican seat held by a former New Yorker, Norm Coleman, looks safer than ever, and Mr. Franken’s hopes could be doomed by his own New York past.

The former “Saturday Night Live” star, best-selling author, and talk show host is lagging far behind in the polls amid a swirl of negative stories about his unpaid taxes and employee benefits, raunchy jokes, and even pornography.

After being endorsed in June by the state Democratic-Farmer-Labor Party, Mr. Franken now faces a primary challenger who says Mr. Franken’s campaign missteps show he is out of touch with Minnesota values and have rendered him unelectable.

Looking to bounce back, Mr. Franken shook up his campaign staff last month and brought in a group of veteran Washington operatives, including a former aide to Senator Schumer and John Edwards, Eric Schultz, and a top adviser to Senator Clinton, Mandy Grunwald.

But political analysts in Minnesota say the damage may be too great. The race, they say, has become a referendum on Mr. Franken rather than the incumbent — an ominous sign for any challenger.

“It’s all about Franken, and it needs to be all about Coleman,” an expert on Minnesota politics at Carleton College, Steven Schier, said.

Both candidates were born in New York City. Mr. Coleman, 58, was raised in Brooklyn and attended Hofstra University in Long Island. He moved to Minnesota after law school and was elected mayor of St. Paul in 1993 as a member of the Democratic-Farmer-Labor Party, the state equivalent of the Democratic Party. He became a Republican in 1997 and won his seat in the Senate in 2002 after the sudden death of the incumbent, Paul Wellstone, in a plane crash shortly before the election.

Though Democrats have criticized him for voting 90% of the time with President Bush, he has developed a reputation as a moderate. He’s also been known for his criticism of the United Nations and has introduced several bills to restrict American funding of the world body.

Mr. Franken, 57, was born in New York City and moved with his family to a suburb of Minneapolis at the age of 4, where he spent his childhood. After graduating from Harvard University, he began a career in comedy and eventually landed back in New York at the then-fledgling “Saturday Night Live” in the 1970s.

After leaving the show in 1995, he penned the best-sellers “Rush Limbaugh Is a Big Fat Idiot and Other Observations” and “Lies and the Lying Liars Who Tell Them: A Fair and Balanced Look at the Right.”

In recent years, Mr. Franken emerged as the star of Air America Radio, hosting a talk show on the liberal start-up network, which struggled to find a foothold. He moved back to Minnesota in 2005 and started a political action committee with an eye toward challenging Mr. Coleman this year.

After announcing his candidacy on his show’s final broadcast in February 2007, Mr. Franken stumped throughout the state, courting labor unions and other key Democratic-aligned interest groups as he sought to prove himself as a serious candidate, and not merely a celebrity on a vanity tour.

By and large, he was successful. Stressing his Minnesota roots, he focused on the issues and followed the national Democratic playbook of trying to tie Mr. Coleman to the increasingly unpopular Bush administration, particularly hammering his support for the Iraq war.

Mr. Franken demonstrated formidable fund-raising prowess and raised more money than any challenger in the country. Activists from the Democratic-Farmer-Labor Party increasingly lined up behind Mr. Franken, and his chief rival for the endorsement bowed out in March.

Though the nomination appeared largely in hand, Mr. Franken’s problems were just beginning. First came the disclosure that he faced $25,000 in penalties for failing to pay workers’ compensation for the corporation he had set up in his name in New York. Then, following a story broken by a Republican blogger, Mr. Franken in April announced that he was paying $70,000 in back taxes and penalties to 17 states. He attributed the error to his longtime accountant, saying he had overpaid the same amount in taxes to New York and Minnesota.

All the while, Mr. Franken continued to be dogged by off-color jokes and writings from his career as a satirist. In particular, Republicans pounced on a sexually explicit parody he wrote for Playboy, titled “Porn-O-Rama.” Also unearthed was a 1995 article from New York magazine, which reported that Mr. Franken once proposed a joke for “SNL” about raping the “60 Minutes” correspondent Lesley Stahl.

Facing criticism from prominent female Democrats in Minnesota, Mr. Franken offered a general apology to the Democratic-Farmer-Labor convention as he accepted the party endorsement in June. “It kills me that things I said and wrote sent a message … that they can’t count on me to be a champion for women, for all Minnesotans. I’m sorry for that. Because that’s not who I am,” Mr. Franken said, according to the Minnesota Star Tribune. “I wrote a lot of jokes. Some of them weren’t funny. Some of them weren’t appropriate. Some of them were downright offensive. I understand that.”

The flaps provided plenty of fodder for Mr. Coleman, who lumped them all together in a television ad featuring three bowlers discussing the campaign. “We’ve read all this stuff about Al Franken,” one says in the ad. “Not paying taxes. Going without insurance for his employees. Foul-mouthed attacks on anyone he disagrees with. Tasteless, sexist jokes. Writing all that juicy porn.” The bowler concludes by saying he and his buddies have decided to run for Senate, because, he says, “we’re just as qualified as Al Franken.”

The spate of negative attention appeared to take its toll with voters. Polls last month gave Mr. Coleman a double digit lead after showing a neck-and-neck race early in the year.

With Mr. Franken’s campaign seemingly sputtering, a little-known St. Paul attorney, Priscilla Lord Faris, announced in July that she would challenge Mr. Franken in the DFL’s September 9 primary. Initially a supporter of Mr. Franken who had contributed to his campaign, Ms. Lord Faris said she concluded he was unelectable.

In an interview, she attributed Mr. Franken’s problems to his long absence from Minnesota. “It’s the total package. We kind of call it the New York City problem,” she said. “The root of it is that he’s been out of touch with Minnesota for so long that he didn’t understand that we don’t talk like that here.”

A DFL spokesman said the party endorsed Mr. Franken “by acclamation” and fully expected him to win. “We’re putting every resource into helping him beat Norm Coleman and take back Paul Wellstone’s seat in November,” the spokesman, John Stiles, said.

The Franken campaign did not return calls for comment.

Though Ms. Lord Faris is not expected to defeat Mr. Franken, she could damage him. She is running television ads criticizing him, and one has already been incorporated into one of Mr. Coleman’s TV spots.

With her entry into the race, Mr. Franken is now hearing the carpetbagger charge from both sides — even from Mr. Coleman, despite his New York roots. “Norm Coleman has been working on behalf of Minnesotans for 30 years. He’s led a Minnesota life,” a campaign spokesman, Mark Drake, said. Mr. Franken “is someone who parachuted in from out of state to run for office.”

Political analysts fault Mr. Franken for not knowing about his tax and employee benefit liability earlier in his bid for office, and they say he was wrong to discount the potential explosiveness of his career as a comedian, a charge the Democrats in Minnesota do not dispute. “He just really believed there was a red line between that past and running for office,” a professor of political science at the University of Minnesota, Lawrence Jacobs, said. “It was naïve on his part.”

Democrats say Mr. Franken’s campaign has improved markedly since the staff additions came on board. He had more than $4 million in campaign cash to spend at the end of the last filing period, and has been running a combination of positive ads along with spots attacking Mr. Coleman. Party strategists say the campaign has succeeded in going on the offensive, and aides have of late been focusing on reports that Mr. Coleman is paying well under market value for an apartment in Washington.

“It’s a very tight race,” said Matthew Miller, a spokesman for the Democratic Senatorial Campaign Committee, which has made Mr. Coleman’s seat a target this fall. He acknowledged that the campaign had suffered a “setback,” but he said Mr. Franken handled it well and noted that the last polls were conducted a month ago.

Mr. Schier of Carleton College said Mr. Franken could come back, adding that Mr. Coleman’s lead was not conclusive. “It’s not over,” he said.