The Senate on Tuesday failed to approve a bill that would keep federal student loans at their current interest rates for another year, once again postponing debate on what has become a controversial issue. The Senate’s party-line vote of 52-45 failed to reach the necessary 60 votes, meaning that 7 million student borrowers could see their rates jump from 3.4 percent to 6.8 percent if the rates are allowed to expire in July. Lawmakers disagree over how to pay for the extension, and it has become a major issue on the presidential campaign trail in the past few weeks.