The governor began his speech by wishing reporters "happy holidays" and "feliz navidad" and said, "I've enjoyed the limelight I've had over the last couple of weeks."
Illinois Lt. Gov. Pat Quinn immediately railed against Blagojevich's announcement, calling his decision to appoint a senator "provocative" and an "insult to the people of Illinois."
"Rod Blagojevich has unclean hands and should not be allowed to make any appointment," Quinn said.
Blagojevich faces impeachment charges by Illinois state legislators, and some say his decision to appoint a senator may also have tactical purposes.
By making the appointment, Blagojevich also removes some of the urgency in impeaching him, said Ken Gross, the former associate general counsel of the Federal Election Commission.
"He wants to show that he can continue performing as governor and do so in honest and lawful way," said Jan Baran, the former general counsel to the Republican National Committee and to President George H.W. Bush's presidential campaign.
Blagojevich's defiant appointment comes just more than a week after his attorney, Ed Genson, said the governor would not fill the Senate seat.
Blagojevich was arrested Dec. 9 on a string of corruption charges, including that he was allegedly peddling Obama's vacant Senate seat to the highest bidder.
The governor's announcement ignited backlash from Democrats and Republicans, both of whom have spent the past month railing against the embroiled governor, including a request by Obama that he resign.
"It is truly regrettable that despite requests from all 50 Democratic senators and public officials throughout Illinois, Gov. Blagojevich would take the imprudent step of appointing someone to the United States Senate who would serve under a shadow and be plagued by questions of impropriety," the Senate Democratic leadership said in a statement today.
White also said he could not certify any Senate appointment made by Blagojevich.
"Although I have respect for Former Attorney General Roland Burris, because of the current cloud of controversy surrounding the governor, I cannot accept the document," White said.
These are not the first warnings from Senate Democrats that Blagojevich has received.
In early December, they implored Blagojevich not to make an appointment and warned him: "Please understand that should you decide to ignore the request of the Senate Democratic Caucus and make an appointment, we would be forced to exercise our constitutional authority under Article I, Section 5, to determine whether such a person should be seated."
The Illinois GOP has been calling for a special election to fill the vacant seat, even forming the Web site FriendsofBlago.com to show the governor's "friends" among Illinois lawmakers.
Illinois House Republican Leader Tom Cross and Illinois Senate Republican Leader Christine Radogno released a joint statement saying that anyone Blagojevich appoints would be "tainted."
Blagojevich has maintained his innocence since federal authorities arrested him earlier this month at his Chicago home on charges that he attempted to sell the vacant Senate seat.
In his only other news conference since his arrest, Blagojevich gave a fiery performance Dec. 19 in which he declared he had "done nothing wrong."
"I will fight, I will fight, I will fight until my last breath," the governor declared.