New Delhi, April 23 -- "Basically we just sort of rack our brains," says Joe Morelle. "What would happen if this happened? Usually the answer is, well, that's never happened before-but this is what we would do." Mr Morelle is the top-ranking Democrat on the House Committee on House Administration. In normal times his team oversees matters of great importance to Congress, such as who gets a parking permit in the House garage. This being an election year, he is preoccupied with an even more existential question. What if Donald Trump tries to steal the midterms?

Mr Morelle's committee is in charge of adjudicating disputed elections in the House of Representatives. He has spent months doing tabletop exercises and dreaming up worst-case scena...