Can the Speaker refuse to swear in someone who won?

Federal law sets the opening-day sequence. Under 2 U.S.C. 25, a Member of the House administers the oath to the Speaker, and the Speaker then administers it to all the Members and Delegates present before they enter on any other business. The Speaker is chosen first, and Members-elect may take part in that election before they are sworn.

Before each new Congress, the Clerk of the previous House prepares a roll of Representatives-elect listing only those whose credentials show they were regularly elected under state or federal law.

When an election certificate is challenged, the House as a whole, not the Speaker or the Clerk, determines whether a member-elect may be sworn in. House Practice states that the House, and not the Speaker or other official, determines whether a Member may be sworn in, and that a certificate of election is not essential to the administration of the oath.

In Powell v. McCormack (1969) the Supreme Court held that the House cannot refuse to seat a duly elected person who meets the constitutional age, citizenship and residency requirements, because it cannot add qualifications of its own. The Court also held that such a refusal can be reviewed in court, and that relief can run against House employees such as the Clerk, Sergeant at Arms and Doorkeeper who carry out an unconstitutional exclusion, though not against members themselves.