The Senate failed Tuesday to advance a bill to keep federally subsidized college student loan rates lower for another year, prolonging debate on an issue that has emerged as an election-year flashpoint. On a party-line vote, senators voted 52 to 45, short of the 60 votes necessary to proceed to debate on the bill. There is no clear path forward at the moment for lawmakers, who have until July 1 to reauthorize lower rates for roughly 7 million borrowers who would otherwise see rates on subsidized student loans jump from 3.4 to 6.8 percent.