Two days ago, a half-dozen Democrats were considered candidates to be the next U.S. Senator from Illinois.

Today, most are likely to need legal representation to answer questions from federal investigators about what they knew of allegations that Illinois Gov. Rod Blagojevich (D) was trying to sell that seat to the highest bidder.

Former federal judge and White House counsel Abner Mikva — mentor to President-elect Barack Obama and other reform-minded Illinois Democrats — said Tuesday, “I would be very concerned if I was a candidate … thinking, ‘What did I say during my interview with [Blagojevich] that could be construed differently than it was intended?’”

The Justice Department on Tuesday arrested Blagojevich and his chief of staff, John Harris, accusing both of a bribery scheme. According to documents filed in the U.S. District Court for the Northern District of Illinois, the FBI recorded telephone conversations in which the two men plotted to trade the open Senate seat for financial considerations that would to benefit Blagojevich. The Senate seat was part of a broader “pay to play” conspiracy involving the governor and other state officials that law enforcement authorities have been probing for years.

Blagojevich’s attorney told reporters in Chicago that the governor “didn’t do anything wrong.”

According to U.S. Attorney Patrick Fitzgerald, Blagojevich was trying to sell the Senate seat that Obama recently vacated. Until Tuesday, Blagojevich was expected to appoint someone in the next few weeks to serve the remainder of Obama’s Senate term, which expires at the end of 2010. Fitzgerald said the governor was in the midst of “a political corruption crime spree.”

Several white-collar lawyers said the announcement of the charges against Blagojevich is likely only the beginning of an FBI investigation into whether the governor actually made or received any offers to exchange the Senate seat for personal benefits.

According to an affidavit filed by the FBI, federal agents eavesdropped on telephone calls beginning Nov. 3 — one day before Election Day — in which Blagojevich discussed his belief that he deserved to get something of value in exchange for appointing a candidate that Obama preferred. The seat is “a f—ing valuable thing, you just don’t give it away for nothing,” Blagojevich allegedly said.

The government alleges that immediately after the election, Blagojevich considered appointing the person that he assumed was Obama’s favored candidate if Obama would make him an ambassador or name him secretary of Heath and Human Services. Although the person is not named in the charges, it appears to be Valerie Jarrett, a Chicago businesswoman and close confidante of Obama who the president-elect has since announced will be a senior adviser in his White House.

Blagojevich and Harris also discussed other candidates and other options for himself, including the possibility of taking a senior position at the union organization Change to Win in exchange for the appointment. The organization said in a statement Tuesday, “No one connected with Change to Win ever considered, discussed or promised any position at Change to Win to Governor Blagojevich, his staff or his advisers.”

By mid-November, the Justice Department alleges, Blagojevich was focusing on the prospect of having Obama gather a few wealthy individuals — including financier Warren Buffett — who would finance the creation of a new nonprofit group that Blagojevich could head up. In one conversation, the government alleges, Blagojevich said he was hoping to get “$10, $15, $20 million in an organization like that,” with the ultimate goal of securing for himself an annual salary of $250,000 to $300,000.

An FBI affidavit filed against Blagojevich does not refer to the candidates and other parties by name, but it offers some hints as to whom some of them may be.

For example, the FBI alleges that Blagojevich leaked information about “Senate Candidate 2” to the Chicago Sun-Times on Nov. 6. The next day, the paper ran a column about state Attorney General Lisa Madigan (D). “The attorney general’s understanding is that she is candidate No. 2 … She has repeatedly expressed that she has no interest in the seat,” Madigan’s spokeswoman Natalie Bauer said Wednesday,

Other Illinois Democrats who have been prominently mentioned as possible candidates for the seat include Reps. Jesse Jackson Jr., Jan Schakowsky and Danny Davis, outgoing state Senate President Emil Jones and Illinois Department of Veterans Affairs Secretary Tammy Duckworth. The government alleges Blagojevich said an associate of one of the candidates offered to raise $1.5 million for the governor for his 2010 re-election in exchange for the Senate appointment.

The government also alleges that Blagojevich was intending to speak with a “President-elect advisor” about the seat and also about the 5th district seat being vacated by Rep. Rahm Emanuel (D), who has signed on as Obama’s chief of staff. It is possible that Blagojevich — Emanuel’s predecessor as the Congressman from the 5th district — was referring to Emanuel, but Fitzgerald made it clear in a news conference Tuesday that he was making no allegations about anyone who was not named in the affidavit.

At a photo opportunity with former Vice President Al Gore in Chicago on Tuesday, Obama said, “Like the rest of the people of Illinois, I am saddened and sobered by the news that came out of the U.S. attorney’s office. But as the investigation is ongoing, I don’t think it would be appropriate to comment further today.”

But as the event was ending, he added, “I had no contact with the governor or his office, so I was not aware of what was happening.”

The affidavit also alleges that Blagojevich was working with a Washington, D.C.-based consultant, but it is not clear who that consultant was.

Fitzgerald said that the investigation is ongoing and that the department filed the charges against Blagojevich to prevent him from appointing a Senator through a corrupt process.

Several defense attorneys said the other parties in the case are all likely to be contacted by the FBI as the government attempts to ascertain whether there were any quid pro quo’s offered in exchange for the Senate seat. Defense attorney Stan Brand said there can be a fine line between political deals that are perfectly legal and an arrangement that the FBI may consider illegal.

“Doing political favors isn’t a federal crime — yet,” Brand said, but “you never know where this stuff is going to go next.”

Shira Toeplitz contributed to this report.