With the Federal Reserve’s MBS portfolio quietly passing the $1 trillion mark in mid-February for the first time since late 2010, Fed Chairman Ben Bernanke this week continued to toe the party line on the historic level of support the central bank has provided to the MBS and Treasury markets. But there is more skepticism from Republicans on Capitol Hill and some Fed board members, and industry analysts are trying to dope out when the Fed buying spree will wind down. “After quadrupling its balance sheet engaging in unprecedented MBS asset purchases and creating an extended negative real interest rate environment, there is...