Miami Mayor Tomas Regalado and Commissioner Marc Sarnoff sued state transportation officials on Monday to stop them from considering a cheaper, plain-vanilla design for a new Interstate 395 overpass downtown in place of a long-promised “signature’’ bridge.

In their lawsuit, Sarnoff and Regalado argue that Florida’s Department of Transportation broke a public promise to replace the obsolete 395 overpass with a dramatic suspension bridge. The goal: to help spur redevelopment along Biscayne Boulevard and mitigate social and economic damage done to adjacent Overtown by the construction of the original span, which split and isolated the mostly black community.

The suit calls the state’s less-ambitious plan for a “segmental box’’ bridge, which is being pushed by FDOT District Secretary Gus Pego, a “bait and switch."

“This is very simple,’’ Regalado said at a Monday news conference in front of the Arsht Center on Biscayne Boulevard, with the existing bridge in the background. “This is about promises not kept.’’

A FDOT spokesman, Brian Rick, said the agency doesn’t comment on pending litigation.

The suit comes just weeks after FDOT officials introduced the stripped-down version of the overpass as a potential alternative following years of planning and some 75 community meetings. Those efforts culminated in the selection by a citizen’s advisory committee of a soaring bridge design, dubbed the Wishbone Arch.

Agency administrators say they have made no decision and have not identified money to build a replacement for the elevated I-395 span, which connects Interstate 95 with the MacArthur Causeway. But they say the plain bridge version, which rests on columns, would cost about $114 million less than the Wishbone bridge, estimated at $673 million. FDOT engineers say the existing span has numerous design, safety and capacity flaws.

Sarnoff and Regalado argue that city officials and neighborhood groups were duped into approving the more elaborate bridge plan, and that they would never have approved the design now being proposed by FDOT. The citizen’s committee unanimously rejected two versions of the simpler bridge when FDOT’s consultants presented them to the group in January.

The lawsuit contends that U.S. law requires FDOT to show its plan has local approval in order to qualify for federal funding for the highway project. Since the local approval was based on a different design, they argue, FDOT’s revised project should not qualify for federal money.

FDOT “baited the public with a ’signature bridge’ and is now switching to a ‘plain Jane’ segmental box bridge’’ the suit says. Erecting the segmental bridge, even an alternative developed by FDOT that’s decorated with a huge lotus-like structure on top, would further blight the area for decades, Sarnoff said.

“This is our one opportunity to make this right,’’ Sarnoff said. “They need to honor their obligation.’’

The suit, filed in Miami-Dade Circuit Court, is seeking an injunction from a judge to force FDOT to adhere to its original design plan. Miami attorney Jay Solowsky, in whose office Sarnoff works, is handling the lawsuit free of charge.

Regalado and Sarnoff filed as individuals because they are seeking a class-action claim on behalf of Miami’s residents, who they say would see their interests damaged by the lesser overpass.

The 1.2-mile I-395 span, which runs along a series of elevated bridges and embankments, has been considered a source of blight since its construction in the 1960s. Both of the replacement alternatives would raise the overpass and remove embankments to allow reconnection of streets blocked by the existing highway, the FDOT says.

Miami Herald staff writer Scott Hiaasen contributed to this report.