Gov. Bruce Rauner and other Republican lawmakers are making a second run at selling and demolishing James R. Thompson Center in the Loop, which they say could add hundreds of millions of dollars to state and local coffers.

The Legislature's GOP leaders, Sen. Christine Radogno and Rep. Jim Durkin, are once again introducing legislation that would allow the state to sell the 16-story, 1.2 million-square-foot structure and move state workers to leased space in other buildings, Durkin said.

The revised plan comes with conceptual renderings of a redeveloped site where the Helmut Jahn-designed building now stands—including one showing a 1,700-foot tower, which would become the city's tallest, on the site.

Rauner's plan faces several big hurdles, starting with the need for legislation just to begin the formal process of seeking a sale of Thompson Center.

Renewed GOP efforts come more than a year after Rauner first announced plans in late 2015 to try to sell the building to a developer that would likely demolish it and replace it with one or more large structures on the full-block site. But that effort never even made it to a vote in the Democrat-controlled Illinois General Assembly.

"I'm hoping this legislative session will be better, period," Durkin said of reintroducing the plan to a new General Assembly.

"The biggest issue we have right now is finding a way to break through the budget impasse," he said. "But there are other things we can do. Being more smart with state properties is one of them. (Thompson Center) has clearly outlived its utility."

This time around, the state's Department of Central Management Services is providing specific estimates of how much a sale of Thompson Center would boost the state's finances. A sale would generate potential net proceeds of $220 million and would prevent the state from addressing $326 million in deferred maintenance on the building, according to CMS.

Its estimates come after a study of Thompson Center by Chicago-based real estate advisory firm RMC International.

"There was no scenario where the existing building, in its existing form, would make sense to keep," said Mike Hoffman, CMS acting director.

Chicago architectural firm Adrian Smith + Gordon Gill Architecture, known for designing some of the world's tallest buildings, created conceptual renderings showing about 3 million square feet of new construction on the site.

One rendering shows a 115-story, 1,700-foot-tall tower that would top the 1,451-foot Willis Tower. It would include offices, retail, residential units, a hotel and an observation deck. A second plan would have buildings of 40, 60 and 70 stories with mixed uses.

The estimated cost to repair Thompson Center is more than triple the $100 million in deferred maintenance that Rauner first cited in 2015. Commercial real estate experts also say the state would be hard-pressed to get even half of its estimated $220 million net proceeds in a sale, since a buyer would still face extensive demolition costs just to clear the site and start a new development.

The process is further complicated by the building's positioning atop CTA train lines and the city's Pedway system. The state also would need to negotiate a buyout of retail space in Thompson Center that is controlled until at least 2034 by Boston-based Winthrop Realty Trust and Chicago-based Marc Realty.

"We've talked with the master (lease) tenant, and we're committed to figuring it out in a collegial way," Hoffman said. "They were open to talks about it."

Marc Principal Gerald Nudo declined to comment, and Winthrop Chairman and CEO Michael Ashner did not return a call requesting comment.

Despite the complications, real estate experts agree a full block in the Loop, if zoned for up to 3 million square feet of construction, would be likely to attract strong interest from potential buyers because downtown development sites of that size rarely become available.

"My expectation is that there would be a high level of interest (from developers) for a full city block," Hoffman said.

Still, a 1,700-foot tall building would be a colossal challenge for any developer. Willis Tower notwithstanding, past proposals for other skyscrapers of similar height have gone nowhere in Chicago, even though they have in other parts of the world.

A sale and redevelopment of Thompson Center also would generate transfer taxes and future property taxes for local government, Durkin said.