When Speaker Nancy Pelosi announced Tuesday that the House would launch a formal impeachment inquiry against President Trump and whether he enlisted a foreign power to help his political aims, she initiated a constitutional showdown without a certain outcome that will likely consume Washington for months to come.

The speaker’s announcement came after revelations that an official in the intelligence community filed a whistle-blower complaint that Mr. Trump had pressured the president of Ukraine to investigate his potential rival in the 2020 election, former Vice President Joseph R. Biden Jr.

The impeachment process going forward will be complicated, involve various congressional committees and will not always have set rules.