TUPELO, Miss. — President Trump on Friday began road-testing his administration’s theory that he is his own best defense against quickly unfolding impeachment proceedings, accusing House Democrats of “plotting to overthrow the election since the moment I won” and targeting his usual lineup of lawmakers, journalists and “deep state” officials he said have conspired against him.

“Yesterday, the Democrats voted to potentially nullify the votes of 63 million Americans,” Mr. Trump said, referring to the 2016 election, adding that Democratic lawmakers were “disgracing themselves and bringing shame among the House of Representatives.”

A day earlier, a sharply divided House voted to endorse an impeachment inquiry into the president, clearing the way for the public phase of the investigation. With House Republicans voting in lock step behind him, Mr. Trump emerged onstage as a one-man war room against accusations of abusing his power and betraying his oath of office by pressuring Ukraine to investigate political rivals.

In Tupelo, the birthplace of Elvis Presley and an industrial town in Mississippi’s northeast, Mr. Trump disparaged Democratic leaders as “mentally violent people” and mocked the testimonies of government officials who have told House impeachment investigators that he had held up aid to Ukraine for political reasons.