The most important issue in mid-term elections is the state of the economy.
One of the most expensive gubernatorial races in the 2014 midterm elections was in Florida
voter turnout is much lower in midterm elections
The Republican Party has taken control of the US House of Representatives on many occasions. Most recently they became the majority in that body following the 2010 elections.
There were a large number of strikes that President Truman had intervened into and caused a lot of complaint. There was an issue about removing wartime price controls. Midterm elections are usually hard on the president and Truman was even less popular than most since he took over for the beloved FDR who left office suddenly in a way that increased his waning popularity.
party identification
There are many social issues that could be the most important to you. Some people think bullying is the most important social issue to them for example.
Incumbents usually win.
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The most important issue for Dekanawidah in the Iroquois Constitution seemed to be peace and non-aggression.
a repudiation of Clinton and the Democrats