Efforts to keep interest rates on new student loans from doubling appeared to be falling apart Wednesday as the Democratic leader of the Senate declared a bipartisan proposal unacceptable.

With just days to spare before a July 1 deadline sends subsidized Stafford loan rates up from 3.4 percent to 6.8 percent, a group of senators from both parties announced a plan that would link interest rates on new federally backed loans to the financial markets. The deal would avert a costly rate hike for now but could spell higher rates in coming years.

The proposal seemed to stall even before it had a chance to be considered.

The chamber’s top Democrat, Sen. Harry Reid of Nevada, said it could never pass. The Democratic chairman of the education panel said he couldn’t back a plan that doesn’t include stronger protections for students and parents.

Undeterred, Sen. Joe Manchin, D-W.Va., said Wednesday he would introduce the legislation today, along with Republican collaborators Sen. Tom Coburn of Oklahoma, Sen. Lamar Alexander of Tennessee and Sen. Richard Burr of North Carolina. Sen. Angus King of Maine, an independent, also signed on to the plan.

Aides to Manchin said he expected to have Democrats on board, as well.

“This deal shows the American people that bipartisanship and common sense are alive in Washington,” Manchin said.

Alexander, the top Republican on the Senate education panel, said: “This proposal is fair to students and fair to taxpayers, and combines the best ideas from the president’s budget, the House-passed bill and the work of this bipartisan coalition of senators. There’s no reason Congress shouldn’t pass it and the president shouldn’t sign it before July 1.”

Republicans have long sought to link student loans to the financial markets instead of letting Congress set the rates for federal lending. President Barack Obama included a variation of that market-based approach in the budget he sent to Congress earlier this year, leaving his fellow Democrats grousing and trying to thwart those efforts.

“Why Senate Democrats continue to attack the president’s plan is a mystery to me, but I hope he’s able to persuade them to join our bipartisan effort to assist students,” said Don Stewart, a spokesman for Senate Minority Leader Mitch McConnell.

McConnell had kept tabs on the Manchin-led talks and GOP aides suggested the resulting proposal might be the best — if not only — way for the Senate to advance legislation that would prevent a rate hike that Congress’ Joint Economic Committee estimated would cost the average student borrower an extra $2,600.

Under the Manchin-led deal, interest rates would be based on the 10-year Treasury note plus an added percentage rate.

There is no limit to how high interest rates could go.

That, Democrats and student groups have warned, will hurt students worse than no deal at all.

“Any proposal that lacks a cap is a nonstarter and indicates that its proponents are putting their ideology above students and their families,” said Allison Preiss, a spokeswoman for the Democratic-led Senate Health, Education, Labor and Pensions Committee that Sen. Tom Harkin leads.