By Nora Eckert and David Shepardson

DETROIT (Reuters) - Nearly 80 members of Congress, including the Senate's top two Democrats, on Wednesday urged automaker Stellantis to honor investment commitments that are part of its contract with the United Auto Workers union.

The union and the Jeep-maker have been in a bitter battle after Stellantis said it would delay a planned investment to reopen an assembly plant in Belvidere, Illinois, though the automaker insists it is committed to the plan and denies it is violating the contract.

The Democratic lawmakers including Senate Majority Leader Chuck Schumer and Senator Dick Durbin cited the Illinois plant delay and other reported decisions by Stellantis.

"These actions violate the obligations Stellantis made to the UAW," the lawmakers wrote to Stellantis CEO Carlos Tavares, asking the automaker, which has come under heat from dealers and shareholders for its declining sales, to lay out a timeline for the investments.

Chrysler owner Stellantis, which recently shook up its top leadership as it tries to turn around slipping North American sales, is now facing another nationwide walkout from the union, just about a year after UAW president Shawn Fain led a six-week strike against Detroit's Big Three automakers.

The company did not immediately respond to a request for comment.

(Reporting by Nora Eckert and David Shepardson; Editing by Chris Reese and Jamie Freed)