House Democrats introduced an article of impeachment against President Trump on Monday for his role in inflaming a mob that attacked the Capitol, scheduling a Wednesday vote to charge the president with “inciting violence against the government of the United States” if Vice President Mike Pence refused to strip him of power first.

Moving with exceptional speed, top House leaders began summoning lawmakers still stunned by the attack back to Washington, promising the protection of National Guard troops and Federal Air Marshal escorts after last week’s stunning security failure. Their return set up a high-stakes 24-hour standoff between two branches of government.

As the impeachment drive proceeded, federal law enforcement authorities accelerated efforts to fortify the Capitol ahead of President-elect Joseph R. Biden Jr.’s inauguration on Jan. 20. The authorities announced plans to deploy up to 15,000 National Guard troops and set up a multilayered buffer zone with checkpoints around the building by Wednesday, just as lawmakers are to debate and vote on impeaching Mr. Trump.

Federal authorities also said they were bracing for a wave of armed protests in all 50 state capitals and Washington in the days leading up to the inauguration.