PHILADELPHIA — U.S. prosecutors took the rare step today to appeal the sentence handed down to long-powerful state Sen. Vincent Fumo in a sprawling corruption case.

Fumo is serving a 4½-year sentence for defrauding the state Senate, a museum and a nonprofit of millions.

Prosecutors had sought at least a 15-year term for the Philadelphia Democrat, who was convicted last year of all 137 fraud and obstruction counts after a five-month trial.

In an appeal filed this morning, they call Senior U.S. District Judge Ronald Buckwalter's decision "unreasonable" and "unduly lenient."

The 67-year-old Fumo was convicted last year of using the victim groups to maintain his mansion, spy on political rivals, pay for yachting vacations and otherwise support his extravagant lifestyle.

"The corruption exposed in this case was breathtaking," prosecutors wrote in filing the 281-page brief. "Defendant Fumo, a 30-year member of the Pennsylvania Senate, used his control of a well-funded Senate committee and of a nonprofit organization he created and supported (Citizens Alliance for Better Neighborhoods), as well as his influence over another nonprofit institution, to support a lavish lifestyle and illegally amass political power."

Prosecutors also appealed the one-year term Buckwalter gave to ex-Fumo aide Ruth Arnao.

The sentences were a fraction of the federal guidelines.