WASHINGTON — 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 percent 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. 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.