Senate Minority Whip Richard J. Durbin on Tuesday said he doubts Speaker John A. Boehner can tackle all of the major items on Congress’ to-do list before stepping down at the end of the month.“All of the things that the House has failed to do, and people are praying Boehner will do before he leaves, are starting to stack up,” Durbin said. “And really it’s an impossible agenda for him to achieve.”