“ But in view of their specific nature, their lack of any necessary relation to the other threats alleged, and the failure of respondent to deny or to account for his failure to deny them specifically, we cannot say that the issue was not one calling for a hearing within the principles laid down in Walker v. Johnston, supra. If the allegations are found to be true, petitioner's constitutional rights were infringed. For a conviction on a plea of guilty coerced by a federal law enforcement officer is no more consistent with due process than a conviction supported by a coerced confession. ”

“ In such circumstances the use of the writ in the federal courts to test the constitutional validity of a conviction for crime is not restricted to those cases where the judgment of conviction is void for want of jurisdiction of the trial court to render it. ”

“ The Government confesses error for the reason that the habeas corpus petition raises the material issue whether the plea was in fact coerced by the particular threats alleged which stand undenied on the record, and that upon that issue petitioner is entitled to a hearing in accordance with Walker v. ”