Washington - The U.S. Senate approved a plan Wednesday to restructure the government's education loan program, tying interest rates to the market and imposing limits on how high those rates can go.

Although supporters are touting the plan as a long-term solution to a difficult problem, some Democratic senators are already saying that they will have to take action on it again soon.

The legislation, which still needs the approval of the House, would dramatically lower interest rates on nearly all new federal education loans taken out by undergraduates, graduate students and parents for the coming school year. But as the economy improves, those rates are expected to increase and could surpass the current rates within five years.

The measure was approved by a vote of 81 to 18, which included 16 Democrats. This passage follows weeks of negotiation and increasing pressure from the White House. Department of Education staffers camped out in senators' offices, and President Barack Obama summoned key negotiators to the Oval Office for a chat. Meanwhile, Republicans in both chambers criticized Senate Democrats for slowing the process.

"At least now they're ready to put their partisan political fix aside and join President Obama and congressional Republicans in enacting real, permanent reform for all students," Senate Minority Leader Mitch McConnell, R-Ky., said.

But this is not a permanent reform, several senators have said, pointing out these concerns: The plan does not address the $1 trillion in student loan debt that already exists. It does not address the growing cost of a college degree. It does not reduce the billions of dollars in profit they say that the government earns from these loans. And it establishes a rate-setting system that will likely lead to higher rates.

"My colleagues who support this proposal say that it will lower interest rates on loans for this year - and that's all that matters," said Sen. Elizabeth Warren, D-Mass. "Now, that's the same thing credit card companies said when they sold zero-interest credit cards, and it's the same thing sub-prime mortgage lenders said when they sold teaser-rate mortgages. In all of these cases, the bill comes due."

Warren introduced an amendment with Sen. Jack Reed, D-R.I., that would cap the new interest rates at the current rates. That amendment failed. So too did an amendment introduced by Sen. Bernard Sanders, I-Vt., that would have authorized the new rates for just two years.

Senate leaders urged their colleagues to vote for the plan, brokered by a bipartisan group, often saying that it's better than doing nothing and can be altered in the future. Sen. Tom Harkin, D-Iowa, leader of the education committee, said Wednesday that "this discussion will continue" next year when the Higher Education Act comes up for reauthorization.

The messages from the White House shifted this week, with a greater focus on the overall cost of college. Education Secretary Arne Duncan said Tuesday that changing the interest rates is just one step in a larger plan. President Obama said in a speech Wednesday at Knox College in Illinois that, in the coming months, he will "lay out an aggressive strategy to shake up the system, tackle rising costs and improve value for middle-class students and their families."

The plan approved by the Senate calls for setting interest rates using the value of the 10-year Treasury bill, plus a percentage add-on. These rates would be locked in for the life of the loan. Undergraduates would have the lowest rates, which could go up to 8.25 percent, higher than the current fixed rate of 6.8 percent.