WASHINGTON (AP) — Millions of college students could be in for a shock this summer when the interest rate on a popular federally subsidized student loan doubles unless Congress acts.

College students on Tuesday delivered more than 130,000 letters to congressional leaders asking them to stop rates from increasing from 3.4 to 6.8 percent. The rate hike affects new subsidized Stafford loans, which are issued to low and middle income undergraduates. They hope to raise enough awareness to get Congress to stop it.

"I will be put back into buying a house and saving up for my expenses later on in life, and life as we know, is very unexpected. Adding that variable definitely limits my ability to be successful," said Tyler Dowden, 18, a freshman at Northern Arizona University who spoke at a press conference outside the Capitol before the letters were delivered in boxes with "Congress: Don't Double Student-Debt Rates" printed on the outside.

Dowden said he anticipates graduating with $25,000 in debt, but if the rate increases, he expects to add about $3,500 to that tally. He's studying to be a mental health therapist.

President Barack Obama frequently tells crowds it's important for Congress to stop the hike because one of the most daunting challenges after high school graduation is affording college. His administration has said keeping the rate low would help 7.4 million borrowers save on average more than a thousand dollars over the life of the loan.

But doing so is estimated to cost billions annually at a time when Congress is gridlocked over budgetary and other issues.

With many lawmakers acting on a campaign promise, the Democrat-controlled Congress in 2007 passed legislation to progressively lower the rate to 3.4 percent this school year.

Rep. John Kline, R-Minn., chairman of the House Education and the Workforce Committee, has said the looming hike is the "result of a ticking time bomb set by Democrats five years ago" and that "simply calling for more of the same is a disservice to students and taxpayers."

Jennifer Allen, a spokeswoman for Kline, said in an email that we, "now face the exact predicament we expected: we must either allow interest rates to rise on student loans, or stick taxpayers with another multi-billion dollar bill."

Kline's office estimates based on a Congressional Budget Office figure that the annual cost to keep the rate low is about $6 billion annually, although some Democrats have estimated it would cost less than that.

With tuition costs at a high, students are taking on unprecedented levels of student debt. College students leave owing an average $25,000 in loans, and student loan debt now surpasses credit card debt.

Some graduates simply can't keep up with it. A report released Tuesday by the federal judiciary about the courts' caseload in the government-spending year that ended Sept. 30, showed that filings with the government as a plaintiff increased 25 percent as cases concerning defaulted student loans surged 58 percent, or by 1,588 cases.

Sen. Jack Reed, D-R.I., and Rep. Joe Courtney, D-Conn., who spoke at the press conference, said it doesn't make sense for student loan recipients to face a higher interest rate than homeowners are getting on mortgages or that banks are able to get. The two back legislation that would keep the lower rate, but both acknowledged that in the political climate, it will be challenging to get the legislation passed by the July deadline.

The college students said they worry constantly about the debt they are taking on, and that the few thousand extra dollars they'd take on in new loans if the rate doubled would affect life decisions.

Samantha Durdock, 19, a sophomore studying government and politics at the University of Maryland, said she wants to go to graduate school after college so that she can pursue a career related to international affairs, but she think she's won't be able to go immediately with the additional amount she'd likely owe. She said she already expects to graduate owing at least more than $20,000.

"Right now, if the loans stay how it is, it's going to be tight, but I could make it doing five years straight in school. But with the increase I'd probably have to take off," Durdock said.