"I firmly believe that changing the rules now, and seating delegates from Florida and Michigan at this point would not only violate the Democratic Party's rules of fairness, but also would be a grave injustice," Sharpton said in a letter to Democratic National Committee Chairman Howard Dean.
"As former presidential candidates we both know that, whether we liked them or not, we adhered to the rules set forth by the Democratic Party to select its nominee for president."
Sharpton, a black activist and radio talk show host, sought the presidency in 2004.
NAACP chairman Julian Bond also wrote Dean recently, taking the opposite position. Bond said failure to seat the delegates would disenfranchise minority voters in Florida and Michigan.
Former U.S. Commission on Civil Rights Chairwoman Mary Frances Berry and former Justice Department official Roger Wilkins also wrote Dean urging the DNC settle the issue before the convention for the good of the party.
Berry - who oversaw the 2001 report that studied the disputed 2000 Florida election and found thousands of voters, particularly black voters, were disenfranchised - said she is also concerned about disenfranchisement of Michigan and Florida voters, although she didn't make a recommendation on how the DNC should resolve the dispute.
The DNC penalized Michigan and Florida for moving their primaries to earlier dates in violation of party rules. Both states were stripped of their delegates, and the party's presidential candidates signed a pledge not to campaign in either state. Florida lost all 210 delegates, including its superdelegates; Michigan, 156.
Since then, waging a hard-fought delegate battle with Sen. , 's campaign has pushed hard for both states' delegations to be seated. Clinton won Florida's primary Jan. 29 and Michigan's Jan. 15, but was the only candidate to appear on the Michigan ballot after the other candidates removed their names.
Sharpton said he disagreed with those who say minority voters in Florida and Michigan will be disenfranchised.
"That claim, if true, should have been made many months ago before the decision was made to strip these states of their delegates, and, once the decision was made, it should have been vigorously objected to and contested by those who felt it disenfranchised voters," Sharpton wrote. "To raise that claim now smacks of politics in its form most raw and undercuts the moral authority behind such an argument."
The DNC has said it would allow both states to hold a different contest, probably a caucus, that would comply with party rules. Either state can also appeal the penalty to the DNC credentials committee, which will not meet again until this summer.