Senate Republicans have blocked a measure that would prevent a doubling of low-interest rates on federally subsidized student loans. The rates on Stafford loans are set to jump to 6.8 percent on July 1, unless Congress intervenes. But on Tuesday, the Senate failed to advance a measure that would extend the lower rate for another year, falling eight votes shy of the 60 votes needed. Republicans want to pay for the extension by cutting nearly $6 billion from preventative healthcare, while Democrats have proposed ending a loophole on payroll taxes.