CHICAGO (Reuters) - Illinois Gov. Rod Blagojevich on Tuesday defied fellow Democrats and appointed a successor to fill Barack Obama’s vacant U.S. Senate seat -- the seat the governor is charged with trying to sell to the highest bidder.

Ignoring warnings from within his own party not to make an appointment, Blagojevich named to the seat former Illinois attorney general and frequent candidate Roland Burris, 71, a fellow Democrat and African-American who lobbied for the post.

The seat vacated by President-elect Obama, a Democrat who was the lone African-American in the chamber, has been in limbo since Blagojevich was arrested by FBI agents on December 9 on charges he attempted to solicit campaign contributions and other political favors.

Blagojevich said he was compelled to act because the Illinois legislature put off consideration of a special election to fill the seat. The governor is fighting impeachment proceedings under way in the state legislature.

“Please don’t allow the allegations against me to taint this good and honest man,” Blagojevich told a news conference.

The governor has denied doing anything wrong and dismissed calls, including from Obama, to resign and give up his sole power to make the Senate appointment, which lasts until 2010.

U.S. Senate Democratic leaders vowed on Tuesday to block the appointment, saying Burris would “serve under a shadow and be plagued by questions of impropriety.”

“It will ultimately not stand,” they said in a statement.

Burris, a lawyer, was the first African-American elected to statewide office in Illinois in 1978 when he became state comptroller, and was state attorney general from 1991 to 1995.

He has run unsuccessfully several times for higher office, including a U.S. Senate bid in 1984 and governor in 1994.

Burris and his consulting firm have donated about $15,000 to Blagojevich’s campaigns since 2002.

“Roland Burris is emblematic of the old-school, pay-to-play culture that has plagued Illinois for generations and this appointment is another embarrassment for the people of Illinois,” state Republican Party Chairman Andy McKenna said.

“It’s a very shrewd political move on the governor’s part,” political analyst Don Rose said. “The Senate has said it won’t accept anybody that he proposes, but here they’ve come up with an African-American with deep roots in the black community.”

The Democrats will be in control of the U.S. Senate with at least 57 seats in the 100-seat chamber when Congress reconvenes on January 6. Still in limbo are Minnesota’s undecided Senate race, as well as Obama’s Senate seat in Illinois.

The U.S. Constitution states that the Senate decides the qualifications of its members. But if the Senate blocked the appointment, the case would likely end up in court.