Is this not an impeachable offense?

Why, yes. Yes it is.

President Donald Trump threatened to declassify “devastating” documents next year if House Democrats launch probes into his administration. “If they want to play tough, I will do it,” he said during an Oval Office interview [...] with the New York Post. "They will see how devastating those pages are." Trump told the paper he would declassify FISA warrant applications and other confidential documents from Robert Mueller’s investigation, which he said would expose efforts by the FBI, the Justice Department and the Clinton campaign to set him up. “If they go down the presidential harassment track, if they want to go and harass the president and the administration, I think that would be the best thing that would happen to me,” he said. “I’m a counter-puncher and I will hit them so hard they’d never been hit like that.” Trump also told the tabloid he would wait to reveal such documents until they are absolutely needed, saying, “It’s much more powerful if I do it then.”

  Daily Beast

...but hey, do what you want...you will anyway.