Final answer:
Campbell's surge-and-decline theory explains that the president's party generally loses seats during midterm elections due to decreased voter turnout among those less politically engaged, who had been mobilized during the presidential election 'surge'.
Step-by-step explanation:
Angus Campbell's surge-and-decline theory suggests that the outcome of midterm elections is characterized by the president's party losing seats in Congress. This phenomenon occurs because presidential elections stimulate increased turnout, bringing in less-politically active voters who often favor the president's party, leading to a 'surge' in support. During midterms, there is a 'decline' in this support, as these less-interested voters are less likely to vote, which subsequently benefits the opposing party.
The theory notes that high-stimulation environments of presidential campaigns tend to decline in midterm years, leading to lower turnout and a shift in electoral advantage. This pattern, though occasionally broken in certain years due to extraordinary events or conditions, has been observed consistently since the Civil War. The 2018 midterm is one such example where the House of Representatives shifted control to the Democratic Party after years of Republican dominance.