Six years after Iowa voters overwhelmingly elected her to the U.S. Senate, Republican Joni Ernst faces a tough challenge from Democrat Theresa Greenfield in a race that has been one of the most expensive in the nation. The two candidates had more than $170 million for spending on media as parties and interest groups poured money into the race, which was eyed as a key battleground for control of the Senate. Ernst breezed into office in 2014 with an 8-point victory thanks in part to an image molded by television ads of her riding a motorcycle, shooting a handgun and promising to make big-spenders in Washington “squeal.”