Comedian Al Franken might have proved he’s good enough and smart enough to be the Democratic nominee for Senate in Minnesota, but are his tax returns?

His campaign announced Tuesday that a team of accountants had uncovered $70,000 in unpaid taxes from Franken to more than 17 states.

Franken is the leading Democratic candidate to face Sen. Norm Coleman (R) this fall, though he must first gain his party’s endorsement in June. Publicly released polls have shown the two candidates running close.

Initially, Democrats were hesitant about a Franken candidacy, given his ties to Hollywood and penchant for speaking off the cuff. But more recently, the former “Saturday Night Live” cast member has shown both Gopher State and national Democrats that he could put together a solid campaign, even forcing his toughest primary competition, wealthy trial attorney Mike Ciresi (D), out of the race.

But the complicated state of Franken’s tax payments, based on the wealth he has accrued as a performer and author, has suddenly become a thorny issue for his campaign — and one that Republicans clearly hope to use against him in the next six months.

Franken overpaid his personal income taxes to his states of residence from 2003 to 2006, Minnesota and New York, and underpaid those taxes to 17 other states where he earned an income, said campaign spokeswoman Jess McIntosh. Because each of these states has different income tax laws, Franken paid $917,000 total in taxes during that time, but should have paid $921,000.

The disclosure of this information, first given to leading news outlets in Minnesota Tuesday afternoon, comes more than a month after Republicans unearthed more than $30,000 in unpaid taxes and penalties by Franken’s corporation in both New York and California.

The breadth of the error, which the campaign blames on Franken’s longtime accountant, however, leaves some political insiders doubting whether the candidate has put together as strong a campaign as some had recently come to believe.

The corporate taxes owed in California and New York are the kind of public information that a research firm could easily have discovered while doing opposition research on Franken.

However, the more recent disclosure on Franken’s personal income taxes owed to 17 states was not public information, McIntosh said. She said Franken and a team of accountants spent last weekend going over his personal tax records from 2003 to 2006 after they discovered an error in the California returns already publicized in the media.

“The GOP probably never would have found it,” she said. “The states were not asking about it. … This was not a matter of public record. It wasn’t something that a self-[opposition] firm would find.”

McIntosh denied that this kind of incident undercuts Franken as a candidate or his ability to run a high-profile Senate campaign.

“None of this addresses the concerns of Minnesotans,” she said. “None of this speaks to Al’s ability to fight for universal health care.”

Democratic Senatorial Campaign Committee spokesman Matt Miller said the flap over Franken’s finances would not influence the committee’s support for him.

“I think the Franken campaign has done a good job in handling this,” Miller said. “He’s taken responsibility, he’s met every obligation and he’s in a position to move forward. … Every campaign will have bumps in the road. This one was in April and now he’s in a strong position to move forward.”

In the meantime, Franken’s disclosure of income tax records not previously available to the public has given Republicans a shot to ask even more questions about his business activity and tax records.

A spokeswoman for the National Republican Senatorial Committee, Rebecca Fisher, said this shows that while voters may think they know Franken, there’s still more they have yet to find out.

“I think what it does is that he may not have been very serious at all about running if there are this many questions out there on the management of his business,” Fisher said.

When the Minneapolis Star Tribune asked Ciresi this week if the news would make him reconsider his candidacy, the 2000 Senate candidate declined to say whether he was interested in rejoining the race.

“I prefer to reserve my comments and judgment until the facts come out,” Ciresi told the newspaper. “At this point, I really can’t predict what the future might bring. You take a look at events and make judgments and decisions.”

Franken’s campaign had no comment in response to Ciresi’s remarks. However, a Washington, D.C., Democratic source said Ciresi has not indicated any interest in running.