U.S. Rep. Jerry Costello doesn't leave office until Jan. 3, but he and his staff are set to leave their coveted suite of offices in the Rayburn House Office Building, in Washington, D.C., by the end of Thursday.

Costello described his impending departure from the U.S. House as a "bittersweet" experience.

"One is that, I'm looking forward to moving on and doing some other things," said the longtime Belleville Democrat, who is finishing up 12 terms representing Southern Illinois in Congress. "And two is that, as I am moving 25 years of records and personal notes from constituents, as well as colleagues and cabinet members, it brings back a lot of memories."

Memories but "no regrets," he said. "It's been a great ride. It's been a privilege and an honor to serve almost a quarter of a century."

Costello recalled that when he was first elected in 1988, he had told friends he did not want to serve the rest of his life in Congress because "there were other things I intended to do. I'm the type of person who looks forward, and not back."

Costello's last official day representing the 12th District is also the same day Congressman-elect Bill Enyart, D-Belleville, is scheduled to recite the oath of office and join the 113th Congress.

Until he retires officially, Costello and his staff will be operating out of a smaller, temporary office in the Rayburn Building. The veteran congressman will still handle constituent concerns, attend the few committee hearings left on his calendar and vote on key legislation.

Costello said he expects one of his last House votes will be on a bill likely to emerge before Christmas as a result of the ongoing negotiations between President Barack Obama and Republican leaders. The measure would center on a solution for avoiding the so-called "fiscal cliff" -- the nickname for the battery of automatic tax hikes and $1.2 trillion in spending cuts set to occur Jan. 1 as part of a plan aimed at reducing the nation's $16 trillion debt.

Many economists, though, have predicted such a one-two punch of tax increases and program cuts could push the nation back toward a recession and send markets plummeting worldwide.

Costello, however, doubts that will happen.

"I do not believe we're going over the fiscal cliff," Costello said. "I believe that, at least most (House) members that I've talked to, understand that it is not an option. So it's hard to predict what's going to happen." Most signs point to the approval of some sort of short-term extension of the Dec. 31 deadline, Costello said.

The short-term "bridge" resolution will not likely raise taxes for middle-class taxpayers, or, as Costello put it, "I doubt that we are going to see a tax increase on 98 percent of the taxpayers in this country. ... I think that it could be postponing the Dec. 31 automatic spending cuts and the tax increases until March or so."

A "comprehensive package" aimed at trimming the nation's budget deficit, he said, "will be for the next Congress."

As for the polarization and gridlock that have plagued the House during his last years there, especially since 2010, when dozens of Tea Party candidates were elected to that chamber, Costello said he's seen encouraging signs of a new spirit of cooperation.

"It is healthy to have vigorous debate ...to hear all parties on each side. But at some point in time those parties have to realize they need to sit down and compromise, give and take, and come up with the best product they can," he said.

"Unfortunately, we have not seen that, especially from those who were elected in 2010. So I hope what I am hearing actually happens. But the proof is in the pudding. We'll have to wait and see."

Costello said he has been in talks with an unnamed university concerning a resting place for papers generated during his years in office. So far he's amassed about 400 boxes of documents. The university in question will likely make its announcement after Jan. 1, he said.

The university, Costello said, "will archive them and have all the public records, documents and so on, available for the public and to students, not only at their university, but for the public as well."

Enyart, a Belleville lawyer and retired commander of the Illinois National Guard, beat Republican nominee Jason Plummer, a Fairview Heights real estate developer, in the Nov. 6 election. Because of nearly $10 million in outside TV attack ads, it was the most expensive congressional election in Southern Illinois history.

As for his plans after leaving office, Costello declined to offer any details, other than to say he wants to spend a lot more time in Belleville with his wife, Georgia, their grown children and their eight grandchildren -- with a ninth due early next year.

"I can tell you this, I've been very flattered with the offers that I've had thus far in Washington and back home," he said. "I know what I want to do is come home and spend time back at home, and if I do anything in Washington, it'll be on a limited basis."