Pension Benefit Guaranty Corporation, like Fannie Mae and Freddie Mac, is a case of "too big to fail." At least a number of members of Congress see it that way. And they are planning a push for legislation designed to shore up underfunded multiemployer private-sector pension funds whose result could put taxpayers on the hook for billions, if not tens of billions, of dollars. Sen. Bob Casey, D-Pa., and Reps. Earl Pomeroy, D-N.D., and Pat Tiberi, R-Ohio, the driving forces behind this measure, seek to shift the primary responsibility of keeping pensions adequately funded from unions and unionized...