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U.S. senators are in for a major fight if they intend to follow through with threats to bar Illinois Gov. Rod Blagojevich's appointment from replacing President-elect Barack Obama in the U.S. Senate next week, say legal scholars, political analysts and even some elected officials.

The current 50-member Senate Democratic caucus had urged Blagojevich not to appoint former Illinois Attorney General Roland Burris to the job while Blagojevich is under the cloud of a possible indictment and impeachment for allegedly trying to sell the Senate seat to the highest bidder. He ignored the request.

Now the entire campaign to de-legitimize Blagojevich is getting harder for U.S. senators and the U.S. attorney handling the case to make, George Washington University law professor Jonathan Turley and others told FOXNews.com.

"There is no question that (Burris) is the legitimate successor to Barack Obama. There is no controversy here as a matter of law. The controversy is purely political. People simply don't like the governor who appointed him," Turley said.

The difficulty in trying to limit Blagojevich's authority was underscored twice in the past two days -- once when Blagojevich defied lawmakers to appoint Obama's replacement, and again when U.S. Attorney Patrick Fitzgerald requested a rare 90-day extension to seek an indictment against Blagojevich.

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Fitzgerald said the holiday season had limited the dates and times that prosecutors could present evidence to a grand jury, and that the investigators still had to review "multiple witnesses" who have come forward as well as thousands of intercepted phone calls. After years of investigation, he said the 30 days usually allotted to bring an indictment against a defendant was insufficient.

The motion seems to conflict with the portrait of the rock-solid case Fitzgerald presented at his press conference on Dec. 9, Turley said.

"I'm surprised by it, because at the press conference he made this out to be the world's strongest criminal case," he said.

Yet Blagojevich holds on to power and still has the legal authority to appoint a replacement -- he retained the authority after the Illinois General Assembly rejected the idea of holding a special election.

Illinois House Republican Leader Tom Cross, who said he was disappointed and "shocked" by Blagojevich's appointment, told FOXNews.com that he anticipates the governor and Burris will fight hard to defend the action.

Already, Burris asked a court on Wednesday to force Illinois Secretary of State Jesse White to certify his appointment. The filing came after White said he would not sign off on Blagojevich's decision -- White spokesman Dave Druker said they had not reviewed the court filing, but the state agency believes it is acting within its authority.

Senate Majority Leader Harry Reid also said he and his colleagues will not seat Burris since he would enter the chamber "under a shadow" and be "plagued by questions of impropriety."

Reid said in a written statement Tuesday that the senators are entitled to judge the appointment because of the concern it is "tainted by fraud." He said they are not judging the qualifications of Burris.

Reid's claim of authority to prevent Burris, who has not been accused of any wrongdoing, from entering the chamber derives from Article 1 Section 5 of the U.S. Constitution. The section says each chamber "shall be the judge of the elections, returns and qualifications of its own members."

The section also says each chamber may "punish its members for disorderly behavior, and, with the concurrence of two-thirds expel a member."

Burris and Blagojevich could potentially cite the decades-old case of Adam Clayton Powell Jr. in defending their legitimacy. Powell sued after the House refused to seat him in 1967 over corruption allegations. Because Powell was qualified and won election, the Supreme Court overruled the House and Powell returned to the chamber.

But Reid's office argued that the Powell case does not apply since they are challenging the integrity of the process, not the integrity of the candidate.

Since Burris has not been sworn in, the first tactic by the Senate may be simply to prevent that from happening, rather than wait until he's sworn in to expel him. But the senators' effort is complicated because Burris technically was appointed through a legal process.

Turley said that Reid would be pursuing a "dangerous precedent" if he tried to block Burris.

"There's no question in my view that this would be an abuse of the Senate's inherent authority."

Carl Tobias, a law professor at the University of Richmond, said one option for the Senate is to seat Burris and then refer the matter to committee for investigation.

"It's all very murky right now," Tobias said.

Tobias said White's objections over certifying Burris seemed like an "enormous technicality" that would probably be resolved. Turley called it an "abuse of power" by the secretary of state, who does not have the authority to make his own individual judgment on the matter if no law was broken in making the Burris appointment.

Both professors said they wouldn't be surprised if the matter headed to court.

The Senate also has a couple key political considerations to make before following through with its threats.

First, with no more than 59 seats, the Democrats are going to be at least one vote short of breaking a filibuster come January. The Minnesota Senate race between Republican Sen. Norm Coleman and Democrat Al Franken also is still unresolved.

Democrats don't want to lose Obama's Senate seat on top of that.

"Every vote counts. Therefore my suggestion to Harry Reid and company, as tough as it may be, it may make sense to take Burris," Democratic strategist Bob Beckel said.

Second, Burris would be the only black U.S. senator if confirmed. U.S. Rep. Bobby Rush stepped in at the press conference Tuesday to urge lawmakers not to reject Burris because of Blagojevich's legal complications.

Marc Lamont Hill, Temple University professor, said the dispute should not be obscured by the issue of race, and Obama, as the first black president-elect, could use his "racial capital" to eliminate race as an issue.

Hill said in any case the Senate will have a tough time denying Burris. Their best option, he said, is to somehow convince Burris to withdraw his name.

"I think this guy's going in," Hill said. "It would be curious in some sense for the senators to allow Al Franken to be a senator and deny Burris."

FOXNews.com's Judson Berger and The Associated Press contributed to this report.