Rep. Ed Case announced Thursday he will challenge fellow Democrat Sen. Daniel Akaka in the September primary, saying voters should have a chance to replace the 81-year-old senator with a younger leader.

Case, 53, said his campaign will not target Akaka's performance but focus instead on the need for a transition that "requires that we phase in the next generation to provide continuity."

Case said he wanted to give voters control over that transition rather than risk having Akaka's successor chosen by Republican Gov. Linda Lingle, who would appoint a new senator if Akaka or Sen. Daniel Inouye died in office.

Akaka has indicated he will run for a fourth term, and no Republican opponent has emerged.

He was not immediately available for comment, and his campaign chairman, Wayne Yamasaki, said he had not talked with the senator about Case, who has served two terms in Congress.

"We welcome challenges because challenges toughen the candidates," Yamasaki said.

Inouye, also 81, said in a statement that he was "rather stunned" by Case's announcement and hoped the congressman would reconsider and instead seek re-election to the House.

He also said Akaka has "the full support" of the leadership of the Democratic Senatorial Campaign Committee.

Inouye was re-elected in 2004.