Congressional lawmakers say it is unlikely they will reach a deal to avoid the doubling of interest rates on government-backed student loans before a deadline next week. Rates for government-subsidized Stafford student loans to low-income students are set to rise on Monday from 3.4 percent to 6.8 percent. The rate would equal the percentage paid by students with unsubsidized loans. The impasse centers around bipartisan differences over whether to cap interest rates on consolidated loans and whether government profits from the loans should go toward paying down the federal deficit. Although lawmakers expect to miss the deadline, Senate Democrats say they will hold a vote as early as July 10. Last month, the Congressional Budget Office forecast a profit of $50.6 billion from the interest it charges students paying back their college debt. Democratic Senator Elizabeth Warren has proposed a measure that would lower student loan interest rates to 0.75 percent — the same rate given to big banks on government loans.