A proposal to have the U.S. Army Corps of Engineers undertake a $3.1 billion construction and restoration project on the upper Mississippi and Illinois rivers is poised for action in the Senate even though it has come under repeated attack from environmental, taxpayer and academic groups -- as well as some administration officials -- as costly and unnecessary.

Sen. Christopher S. Bond (R-Mo.), who chairs the subcommittee on transportation and infrastructure, and other midwestern senators are trying to write the plan into a broader water project authorization bill this month, the first time it has been formally proposed. Bond said he hopes to move the bill through the Environment and Public Works Committee before the July 4 recess, though a senior GOP aide said it was hard to predict whether the bill would pass before the Senate's fall adjournment.

The proposal, which would be the most expensive navigation project in U.S. history, has bipartisan support from Sen. Tom Harkin (D-Iowa) and others, as well as the backing of agricultural producers and barge operators. But the White House has yet to endorse the plan, and some senators have vowed to fight it.

The Corps will begin holding public hearings on the project next week, and its officials said they will hold off on a recommendation on construction until they solicit more public comment. But Bond is not waiting for the Corps, saying, "It's time Congress chose to chart the future based on the information we have."

Bond's bill, which is similar to the Corps' pending plan, would include $1.46 billion for ecosystem restoration along the rivers and would draw about $850 million from a trust fund supported by a tax on barge operators. It differs in not calling for doubling the size of five 600-foot locks on the upper Mississippi and Illinois rivers.

Bond's initiative marks the latest chapter in a battle between the Corps and its critics. The agency has spent the past few years defending itself against criticism that it manipulated data to justify ambitious projects on the nation's rivers.

After more than a decade of studies costing $70 million, the Corps described the $1.4 billion construction of seven locks, with the option to expand five more, as the "preferred alternative" for reducing river traffic congestion on the upper Mississippi and Illinois rivers. Locks operate as a sort of staircase on the river, raising or lowering vessels so they can meet the river's level as they move upstream or downstream; delays at each lock can last several hours.

"We don't think you can afford to stand still, given that traffic will increase and delays will increase," said Richard Worthington, a senior policy analyst at the Corps. "We recognize there's uncertainty. But there's a great risk in doing nothing."

Bond said the project to remake the 70-year-old lock system is "vitally important to farmers in Missouri and the Midwest. If we don't have good river transportation, particularly when Brazil and other countries are building up their systems, we're going to lose out on a good portion of a $60 billion export market for agricultural products."

The Agriculture Department's Agricultural Marketing Service has endorsed the plan, praising the Corps for working "to provide a modern, efficient and more competitive transportation system for grain exports."

But several independent groups question whether the government should invest so heavily in a navigation project when traffic on the Mississippi has been declining. According to Corps statistics, barge traffic at Lock 25, one of the busiest on the river just north of St. Louis, declined 23 percent between 1992 and 2003. This year it has dropped 19 percent.

"If you're going to justify some new locks, you have to have some increase in traffic," said Lester B. Lave, an economics professor at Carnegie Mellon University who studied the project for the National Academy of Sciences. He said it "takes time" to determine whether the project is justified, adding, "Apparently Senator Bond is impatient."

Debate over the project has raged for years. The Corps asked economist Donald C. Sweeney II, to study the plan in 1993; when he concluded five years later that the $1 billion cost of the proposed project would far outweigh its benefits, Corps leaders yanked the study away from him, replaced him with an engineer and continued to push for lock expansion. Sweeney filed for whistle-blower protection, and a U.S. Army inspector general report later concluded "one of the key parameters was manipulated to result in a specific study outcome."

Two National Academy of Sciences studies over the past four years, as well as the president's Office of Management and Budget, have questioned the methods of analysis used by the Corps of Engineers. The Corps wrote in April that with enough maintenance, "the navigation structures on the system could keep functional for the next 50 years."

Even Corps officials have raised questions about the data underlying the plan, according to internal correspondence obtained by The Washington Post. Arlene Dietz, director of the Navigation Data Center at the Corps' Institute for Water Resources in Alexandria, sent an e-mail May 14 to her colleagues in which she questioned whether the locks' capacity had been adequately studied in light of industry complaints about congestion and delays.

"My Greatest Concern is twofold, first we have not adequately evaluated the accuracy, completeness and consistency of the data, and second, our analysts have not been able to spend sufficient time evaluating the trends and relationships," she wrote.

Scott Faber, a water resources specialist at the advocacy group Environmental Defense, said the project "is not only a referendum on the Mississippi, it's a referendum on whether the Corps has to conduct credible studies. . . . If they can cheat here, they can cheat anywhere."