If President Trump asked then-FBI Director James Comey to drop the criminal investigation of General Michael Flynn, this would constitute obstruction of justice under 18 U.S.C. section 1505. Section 1505 makes it a crime to "endeavor to influence, obstruct, or impede" "any pending proceeding … before any department or agency of the United States." Obviously, Trump had knowledge that Flynn was the target of an FBI investigation. The FBI investigation was a "pending proceeding . . . before [a] department or agency of the United States." Further, if Trump had knowledge of a pending grand jury investigation targeting Flynn, his conduct would constitute an attempt to influence or obstruct a grand jury investigation. The FBI was an active participant in the grand jury investigation.

Also, 18 U.S.C. 1512(c)(2) punishes "Whoever corruptly . . . obstructs, influences, or impedes any official proceeding, or attempts to do so." A violation of section 1512 imposes a maximum sentence of 20 years

18 U.S. Code Chapter 73 - OBSTRUCTION OF JUSTICE | US Law | LII / Legal Information Institute