In Iowa, Ernst and Braley each have the support of 43 percent of registered voters. But both candidates remain unknown to many in the state; 14 percent of voters are undecided about who they support in the race, and about a third say they are unsure about their opinion on the candidates or have never heard of them.

But both candidates are polarizing to the voters who know them. Braley gets positive marks from 36 percent of voters and a negative rating of 32 percent. Ernst’s fav/unfav rating is 38 percent to 33 percent.

“Any way you slice the numbers, you end up with a close race,” said Lee Miringoff, the director of the Marist College Institute for Public Opinion, which conducted the poll for NBC News. “This is one that bears watching as the campaign unfolds.”