When the same party controls both the U.S. presidency and Congress, ambitious legislation can happen. Look at the Affordable Care Act, known as Obamacare, passed under Democrats in 2010, or the bill that cut $1.5 trillion in taxes that was passed by Republicans at the end of 2017. The congressional elections coming on Nov. 6 could bring one-party government to a halt -- and potentially spell all sorts of problems for President Donald Trump, a Republican -- if it’s a “wave election” that breaks in favor of the Democrats.
There’s no precise definition, but the term is used to describe when a political party makes major gains in the House of Representatives, the Senate, or both. Generally speaking, congressional wave elections have occurred at the midpoint of a president’s four-year term -- when the president isn’t on the ballot -- and have usually benefited the party that doesn’t occupy the White House, which right now is the Democratic Party. And in a wave election, the closest races tend to break one way.