In interviews, more than two dozen lawmakers and strategists described the meticulous efforts.

Little was left to chance: Republican operatives sent fake campaign trackers — interns and staff members brandishing video cameras to record every utterance and move — to trail their own candidates. In media training sessions, candidates were forced to sit through a reel of the most self-destructive moments of 2012, when Todd Akin and Richard Mourdock’s comments on rape and pregnancy helped sink the party.

Other factors affected the outcome, of course. The election was unfolding during perhaps the best political climate Republicans had seen since the 1980s. Upheaval on the domestic and international stage — a crash of the health care website, beheadings in the Middle East, a surge of migrant children along the Mexican border and a raging virus in Africa — all helped tip the scales in Republicans’ favor. Democrats battled to keep the most competitive races from slipping away from them until the very last minute, an almost impossible task given President Obama’s low approval ratings and the cascade of bad news that was unimaginable when the party was riding high a year ago, after Republicans stumbled through a government shutdown.

“There wasn’t a moment this cycle where we thought, ‘Oh, we can’t lose,’ ” said Guy Cecil, executive director of the Democratic Senatorial Campaign Committee. “Conversely, there wasn’t a moment in this cycle where we thought we couldn’t win,” he added, insisting that their plan was the right one. “Election outcomes tend to declare everyone either a genius or a failure, but there’s no question in my mind that this was the right strategy.”