With control of the Senate up for grabs in 2016, Senate Republicans have spent the year trying to prove Mr. Frank wrong. And Senator Mitch McConnell of Kentucky, the majority leader, believes they have been pretty good at it — particularly when measured against the vicious stalemate of the previous four years. “I think the Senate is functioning again and producing results,” Mr. McConnell said in an interview as he assessed 2015 and looked ahead to a challenging election cycle that could quickly end his control of the Senate agenda if the public disagrees with his take. Mr. McConnell can tick off the bills he sees as victories — a budget, a long-sought solution to a perennial problem with Medicare doctor fees, the first changes to Social Security in decades, a cybersecurity bill, a reconciliation measure undercutting the health care law, Keystone XL oil pipeline approval, the Iran nuclear review law and, most recently, major transportation and education bills. He admits he could not have compiled those achievements without significant help from an unlikely quarter — the Senate Democrats he so frustrated in his position as minority leader.