The US Senate race in Wisconsin may be the number one race to watch in the country, according to the Huffington Post.

Ask a Democrat what Senate races worries them this year, and Missouri, Indiana, West Virginia and North Dakota will likely be near the top. All are states that Donald Trump won in 2016, and the Democratic senators seeking re-election in them have staked out moderate paths to appeal to some of their less liberal constituents.

But increasingly, Democrats point to another race as one they’re worried about: Wisconsin. That’s where Democratic Sen. Tammy Baldwin already has been targeted by an onslaught of outside spending from conservative groups. Party leaders say the odds remain in her favor, but they’re sure not going to take the race for granted.

Conservative groups have so far spent at least $3.1 million against Baldwin, which is more than what all the other Democratic Senate incumbents on the ballot this year have faced combined (under $550,000), according to the Center for Responsive Politics.

Aside from the money, part of the reason Baldwin is vulnerable is that she is running for re-election at the same time Wisconsin Governor Scott Walker (R) is also running for re-election. Walker, according to most outside observers, has a terrific voter turnout machine and is a formidable candidate.

“Scott Walker’s got a great operation,” a GOP campaign aide told the Huffington Post while predicting that the Senate race will be tight. The governor has “a fully formed, aggressive, well-funded effort already underway in every major voting population center in the state, and the nominee (to oppose Baldwin) will benefit from it.”

The Huffington Post also compared the two GOP candidates running to take on Baldwin.

On Delafield businessman Kevin Nicholson:

Nicholson is an untraditional GOP candidate, to say the least. Sure, he’s a clean-cut veteran of the Iraq and Afghanistan wars with degrees from Ivy League universities who expounds on conservative ideas. But he’s also the former president of the national College Democrats who spoke at the 2000 Democratic National Convention. Supporters say he’s a fresh face who came around to the power of conservative ideas, while detractors say he’s an opportunist who can’t be trusted.

On state Sen. Leah Vukmir (R-Brookfield):

Challenging Nicholson for the Republican nomination is Leah Vukmir, a conservative member of the state Senate who has won endorsements from powerful Wisconsin GOP politicians and praise from talk radio hosts. Vukmir is backed by Wisconsin billionaire Diane Hendricks, who also pledged her support for the state senator before she even entered the race.

The political news website Axios explains why the Huffington Post report is significant:

There are 10 Democratic senators up for re-election this year in states that Trump won. But if conservative groups spending is any indication, Wisconsin might be the hardest seat for Democrats to keep — and that should have the party worried at every level because Sen. Baldwin is one of two Democrats serving statewide in Wisconsin.