WASHINGTON — President Trump trained his rancor on federal investigators on Monday and appeared to warn that negative material would emerge about the prosecutors leading the inquiry into Russia’s interference in the 2016 election.

In a series of Twitter posts on Monday, the president said the investigation was being run by Democrats and raised the question of whether the special counsel’s work was being drawn out to influence the outcome of the coming midterm congressional elections, when Democrats could win enough seats to take control of the House and possibly the Senate.

“Just wait ’till the Courts get to see your unrevealed Conflicts of Interest,” Mr. Trump wrote in one of the early-morning tweets.