Franken, a well-known comedian and writer, trailed Coleman by a thin margin after the Nov. 4 election. After a statewide hand recount, Franken edged ahead by 225 votes out of 2.9 million cast.
The Minnesota state canvassing board's decision to call Franken the winner would bring the Democrats' total number of senate seats to 58, with President-elect Barack Obama's seat still in limbo due to corruption allegations against Illinois Gov. Rod Blagojovvich.
Franken was declared the winner shortly after the Minnesota Supreme Court rejected Coleman's appeal to have hundreds of rejected absentee ballots considered in the recount, the Minnesota Star Tribune reported.
Coleman's campaign responded Monday saying it would file a lawsuit claiming the ballots were wrongly rejected.
"Given our campaign's unwavering commitment to ensuring that the vote of no Minnesotan is disenfranchised, today's ruling by the Minnesota Supreme Court is both disappointing and disheartening," Coleman recount attorney Fritz Knaak said in a statement.
According to the Associated Press, Coleman has seven days to file a lawsuit challenging the result of the recount. A lawsuit would prevent Franken from taking the seat in Washington until it is resolved.
The Franken campaign praised the recount process, the Star Tribune reported.
"Today, the Supreme Court once again affirmed the validity of the rules under which this recount was conducted," said Franken recount lawyer Marc Elias.
Despite the reports of a Franken victory, Minnesota Secretary of State Mark Ritchie cautioned the canvassing board's certification of the recount was an endorsement of the recount numbers and not a declaration of a winner.
"We're not doing anything today that declares winners or losers or anything to that effect," Ritchie said.
New York Sen. Charles Schumer, who recently led the Democratic Senatorial Campaign Committee, was ready to make a declaration, saying the race is over and the seat belongs to Franken.
"With the Senate set to begin meeting on Tuesday to address the important issues facing the nation, it is crucial that Minnesota's seat not remain empty, and I hope this process will resolve itself as soon as possible," Schumer said.
Texas Sen. Jon Cornyn, head of the National Republican Senatorial Committee, criticized Schumer's statements because Schumer now controls the Senate Rules Committee.
"Prejudging the outcome while litigation is still pending calls into question his ability to impartially preside over this matter when it comes before the committee, as it most certainly will," Cornyn said, according to the AP.
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