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He could be the toughest opponent to the front-runner to the GOP nomination, state House Speaker Thom Tillis (R). Tillis begins the race with a huge cash advantage and establishment support, however, and it's unclear whether Harris will be able to raise the money to be competitive in the race.

Tea Party-backed Dr. Greg Brannon (R) is also in the race, and announced he raised $150,000 in the last quarter, enough to make some noise in the primary.

Harris has long been expected to run against Sen. Kay HaganKay HaganDemocrats can win North Carolina just like Jimmy Carter did in 1976 North Carolina will be a big battleground state in 2020 Linking repatriation to job creation MORE (D-N.C.), He started a "listening tour" on the race early last summer and told supporters a month ago that he was likely to run.

The race is key to GOP hopes to retake the Senate — many Republicans believe whoever wins North Carolina will hold Senate control.