WASHINGTON — President Trump was watching television in the White House on Wednesday morning when cable news channels started airing Speaker Nancy Pelosi and Representative Adam B. Schiff, the chairman of the House Intelligence Committee, warning at a news conference that any attempts by the president to stonewall their impeachment investigation would be viewed as obstruction.

Mr. Trump did not wait for Ms. Pelosi and Mr. Schiff to finish before responding. First he attacked Ms. Pelosi on Twitter, saying she was neglecting the work of Congress “and trying to win an election through impeachment.” Then he tweeted again, sharing a campaign video that accused Democrats of trying to undo the results of the 2016 election.

He continued those attacks later in the afternoon, both before and after a meeting with Sauli Niinisto, the president of Finland, and became increasingly angry as he went on.

Mr. Trump has long believed that he is the best communicator in the White House, but as the presidential campaign picks up its pace and the prospect of his impeachment becomes more real, he seems to be its only empowered communicator, a one-man war room responding to developments almost hour by hour. And that is making many Republicans anxious.