MADISON, Wis. (AP) — Democratic U.S. Sen. Tammy Baldwin has raised more than $3.7 million over the first three months of the year, more than triple one of her Republican opponents.

Baldwin's campaign reported the fundraising total on Tuesday. She raised nearly $1 million more between January and March than the $2.8 million she collected in the last three months of 2017.

Republicans Kevin Nicholson and state Sen. Leah Vukmir (VOOK'-meer) are running to take on Baldwin. Nicholson on Monday said he had raised more than $1 million in the quarter. Vukmir has not released her fundraising totals.

The Center for Responsive Politics says outside groups have already spent nearly $3.1 million against Baldwin and a whopping $4.8 million in support of Nicholson, with another $909,000 for Vukmir. Just over $1 million has been spent for Baldwin by outside groups.