Now that the House has voted to impeach President Trump, the matter would normally move to the Senate for a trial — the third for an American president.

But questions about the timing of a trial arose after the House approved two articles of impeachment against Mr. Trump. Nancy Pelosi, the House speaker, wouldn’t say when she would transmit the articles, indicating that she may wait to get certain assurances about the fairness of a trial in the Republican-controlled Senate.

A trial would require a two-thirds majority — 67 senators — to convict and remove the president. Republicans control the Senate, 53 to 47, and Mr. Trump is widely expected to be acquitted. But Democrats have argued that impeaching him is a moral necessity, even if he remains in office.

Mr. Trump is only the third president to face a Senate trial for “high crimes and misdemeanors.” (The first two, Andrew Johnson and Bill Clinton, were both acquitted. Another president, Richard M. Nixon, resigned rather than face impeachment and trial.)