Senators Ted Cruz of Texas and Tommy Tuberville of Alabama planned to object on Wednesday to the certification of Arizona’s electors; Senator Kelly Loeffler of Georgia intended to object to those from her state; and Senator Josh Hawley of Missouri planned to object to Pennsylvania’s slate, according to people familiar with their plans.

Their challenges were all but certain to fail amid bipartisan opposition. But their decision to join House Republicans in seeking to overturn the election ensured that Congress would be thrust into a caustic debate over the results and Mr. Trump’s repeatedly debunked claims of widespread fraud and irregularities that could last nine hours or more…

Around Capitol Hill, the mood was tense as lawmakers and their aides tried to game out eventualities that neither party has had to seriously consider in more than a century. With supporters of Mr. Trump already massing in Washington to protest the outcome, the police erected a protective security perimeter around the Capitol and advised lawmakers to use a network of underground tunnels to get to and from their offices on Wednesday.

Just outside the perimeter, a group of around 20 constituents clad in Trump gear surrounded Senator Kevin Cramer, Republican of North Dakota, and pressed him to explain his decision to accept the results of what they called a “fraudulent” election. Mr. Cramer stood firm, saying he could not in “clear conscience” go against his reading of the Constitution to give another term to Mr. Trump. Several people in the group left downcast, but not before holding their hands out to Mr. Cramer to offer a prayer that the country not “crumble.”