By Lisa Baertlein, David Shepardson

LOS ANGELES/WASHINGTON (Reuters) -A group of Democratic U.S. lawmakers vowed not to intervene if contract talks between United Parcel Service and the Teamsters Union fail and 340,000 workers go on strike on Aug. 1, risking billions of dollars in damage to the nation's economy.

Twenty eight senators including Bernie Sanders and Sherrod Brown will respect the rights of workers "to withhold their labor and initiate and participate in a strike," the senators wrote in a letter to the leaders of world's largest package delivery company and the International Brotherhood of Teamsters on Wednesday.

Nevertheless, those lawmakers and 172 representatives including labor caucus co-chairs Representatives Debbie Dingell, Steven Horsford, Donald Norcross and Mark Pocan, in separate letters urged a deal before the current contract expires at midnight on July 31. All the signers are Democrats or independents that caucus with Democrats.

The lawmakers said Congress in recent history has not implemented a collective bargaining agreement between workers and their employer under the National Labor Relations Act. The lawmakers said they were committed to that tradition with respect to the UPS-Teamsters talks.

A key sticking point in the talks is pay increases for experienced part-time workers who are making roughly the same or even less than new hires because starting wages jumped due to the labor shortage in the last few years.

Teamsters leader Sean O'Brien told Reuters last week he has asked President Joe Biden not to intervene in the talks, even as retail groups and other interested parties push for the administration to weigh in.

Any disruption to the business of UPS would be broadly felt because the company handles about 20 million packages a day - about quarter of the parcel shipments in the United States. Those include deliveries for online retailers like Amazon.com, high-value prescription drugs for doctors and hospitals, and inventory for millions of other large and small businesses.

A strike could be one of the costliest in at least a century, with the impact of a 10-day strike topping $7 billion, according to one think tank.

UPS pilots, who belong to a different union, would also stop flying in solidarity with the striking workers.

A U.S. retail industry group earlier on Wednesday urged UPS and the Teamsters union to reach a labor contract deal and avert a strike.

"Even the most robust planning won't shield retailers or consumers from the impact of shutting down a key component in the supply chain as we head full-steam into back-to-school and then holiday shopping seasons," the Retail Industry Leaders Association (RILA) said.

(Reporting by Lisa Baertlein in Los Angeles and David Shepardson in Washington; Editing by Chris Reese and Josie Kao)