MIAMI, Okla. — On the shimmering waters of Grand Lake, a popular vacation spot in northeastern Oklahoma, families have spent the summer splashing around in boats, fishing for the lake’s famous bass and enjoying weekend getaways at upscale waterfront homes.

But drive just a few dozen miles north, and the festivity dissolves into fear over flooding in Miami (pronounced My-am-uh), a city of 13,000 where one in four people live in poverty. For years, the town has fought a losing battle against the wealthy community at the lake, where high water makes for better boating but leaves little room for overflow when it rains. With heavy rains this year, the city of Miami and local Native American tribes say they were again left to pay the price when floodwater clogged upstream, damaging their homes, businesses and ceremonial grounds.

Now, this backyard battle has escalated to the halls of Congress, after one of the lake’s residents, Senator James M. Inhofe, got involved. After decades of debate, local leaders had pinned their hopes on a rare chance to ask a federal agency to help stop the flooding. But Mr. Inhofe — a top Republican who is known to swim and fly planes around the lake, where his family owns a vacation home — quietly introduced legislation in June that would hamstring that agency.

He added the protections in an amendment to the National Defense Authorization Act, a military funding bill that is up for consideration before Congress. As chairman of the Senate Armed Services Committee, Mr. Inhofe has wide latitude over provisions included in the 1,700-page bill.