MADISON, Wis. — The parents of Republican U.S. Senate candidate Kevin Nicholson have donated the maximum amount to the campaign of their son's Democratic rival, Sen. Tammy Baldwin.

Nicholson, a Marine Corps veteran who served in Iraq and Afghanistan, is in a primary battle against Republican state Sen. Leah Vukmir, with the winner advancing to take on Baldwin in the fall.

Federal records show that Nicholson's parents, Michael and Donna Nicholson, both gave $2,700 in December to Baldwin. Nicholson is a former Democrat, but his parents have a long history of donating to Democratic candidates. CNN first reported the donations to Baldwin.

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Nicholson says in a statement that "My parents have a different worldview than I do, and it is not surprising that they would support a candidate like Tammy Baldwin who shares their perspective."

Nicholson says he is a conservative by choice "not because I was born one." He's running as a fiscal conservative who promises to rein in spending and calls the government's debt and deficit "immoral" on his campaign website. Nicholson also says that the greatest threat to U.S. national security is the quickly-growing national debt.