U.S. Chief Justice John Roberts, who cast the decisive vote in 2012 to beat back the first major challenge to President Barack Obama's health care law, kept his cards close on Wednesday.

And the man who occupies the center chair on the Supreme Court's mahogany bench and often dominates arguments seemed to remain deliberately inscrutable.

Roberts asked few questions, none revealing his view of the challenge to the crucial tax-credit subsidies that help low- and moderate-income people buy insurance under the 2010 law. His first question came late in the arguments of the challengers' lawyer and was, in fact, more of a joke.