In Krulewitch v. United States, the U.S. Supreme Court ruled on a case involving conspiracy and hearsay evidence. The defendant, Samuel Krulewitch, was convicted of conspiring to commit offenses against the United States by transporting a woman across state lines for immoral purposes (a violation of the Mann Act). During his trial, statements made by his alleged co-conspirator were admitted as evidence under an exception to hearsay rules that allows such statements in cases where there is independent proof of a conspiracy. However, this "independent proof" was not presented until after these statements had been heard by the jury. The Supreme Court reversed Krulewitch's conviction with a 5-4 decision stating that it violated his Sixth Amendment right to confront witnesses against him because he could not cross-examine those who made out-of-court declarations implicating him in the crime. Furthermore, they held that admitting such hearsay testimony before establishing clear evidence of conspiracy risked prejudicing jurors' minds unfairly against defendants like Krulewitch.

In the dissenting opinion for Krulewitch v. United States, Justice Jackson argued that the majority's decision to uphold Mr. Krulewitch's conviction was a departure from established legal principles regarding hearsay evidence and conspiracy charges. He contended that the court had allowed inadmissible hearsay evidence to be used against Mr. Krulewitch, which unfairly prejudiced his case and violated his rights under the Sixth Amendment of the Constitution - specifically, his right to confront witnesses against him. Furthermore, he criticized how broadly conspiracy laws were being applied in this case and others like it; he believed such broad application could lead to abuses of power by prosecutors who might use these laws as a means of securing convictions when other charges would not stick or when direct evidence was lacking.