Critical elections in the united states typically have occurred

They generally occur at the time when groups of voters begin to change their views on traditional patterns of party loyalty.

Explanation:

Critical elections is a term used in politics to show a feedback, a change in the party ideology of a group of people who are usually party leaders or politicians. This ideological change refers to the changes of opinions on some concepts defended by the party that a certain politician is part of, generating a disagreement with between the politician and those concepts.

This change of opinion results in an intense change in the political structure of a region.

In summary, we can say that critical elections occur when they usually occur at the time when groups of voters begin to change their views on traditional patterns of party loyalty.