(CNN) Mitt Romney is an anomaly in American politics, a former presidential nominee who has now set his sights on a lower office.

He tried and failed twice to win the White House, rising in 2012 to be Republicans' choice on the general election ballot, which is usually the end point of an electoral career.

There are exceptions -- sort of.

John McCain didn't exactly fade into the woodwork after losing to Barack Obama in 2008. He kept his job in the Senate after failing to win the White House in 2008. McCain ran for reelection in 2010 and 2016. So did John Kerry, who ran for reelection to the Senate in 2008 and also went on to be Obama's secretary of state.

But there's a difference between staying put in the Senate or Congress and actively seeking a new electoral role.

Read More