WASHINGTON—House Speaker Mike Johnson (R-La.) has only one goal on the first day of the new Congress: keeping his job and tamping down a potential Republican mutiny. On Jan. 3 at 12 p.m. ET, the 119th Congress will have its first sitting. Its first order of business is to choose a speaker: No other official business, including the Jan. 6 certification of President-elect Donald Trump’s election victory, can take place prior to the election of a speaker. In order to keep the gavel, Johnson will need to win a majority of votes in the full House, though the threshold varies based on how many seats are vacant and how many lawmakers actually take part in the vote….