When Chief Justice John G. Roberts Jr. takes his usual center seat on the Supreme Court bench on March 26, he will begin presiding over an extraordinary three days of arguments that will determine the fate of President Obama’s sweeping health care law.

The decision in the case, expected by June, will have practical consequences for tens of millions of Americans without health insurance, and it may affect Mr. Obama’s re-election chances.

It will also shape, if not define, the chief justice’s legacy.