Colorado: Cory Gardner and John Hickenlooper

Republican Sen. Cory Gardner is running for a second term and faces one of the toughest roads to reelection. Former Democratic Gov. John Hickenlooper is the national Democrats’ preferred candidate, but he faces a June primary against Andrew Romanoff, a former speaker of the Colorado state House, and at least one other Democrat.

Republicans have booked nearly $12 million ($5.5 million from Senate Leadership Fund and $6.4 million from the NRSC). Senate Majority PAC booked $5.2 million here; the DSCC has not booked anything here so far.

Iowa: Joni Ernst and Theresa Greenfield

Republican Sen. Joni Ernst is running for a second term in a state seen as an emerging battleground. National Democrats have endorsed Theresa Greenfield in the June primary over other Democrats Michael Franken, Eddie Mauro and Kimberly Graham. Both parties have already invested significantly in this race for the fall.

Republicans have booked more than $15 million so far ($12.6 million from Senate Leadership Fund and $2.6 million from the NRSC). Democrats have booked more than $20 million ($13.1 million from Senate Majority PAC and $7.3 million from the DSCC).

Kentucky: Mitch McConnell and Amy McGrath

Majority Leader Mitch McConnell is running for a seventh term six years after he helped orchestrate Republicans’ takeover of the Senate. Democrat Amy McGrath is the favorite to face him, but she is running against several other less well-funded candidates in a June primary.

Republicans have booked $10.8 million in this race through super PAC Keep Kentucky Great, which is affiliated with Senate Leadership Fund. Democrats have not booked any money for this race yet.

Maine: Susan Collins and Sara Gideon

Republican Sen. Susan Collins is seeking a fifth term, and is likely to face the toughest reelection of her career. Democrat Sara Gideon, currently the Maine House speaker, is the favorite to earn the nomination against her in the July primary. Both Gideon and Collins have already spent millions on TV, and both parties are invested heavily in the fall.

Republicans have booked more than $12 million so far ($7.2 million for Senate Leadership Fund and $5.1 million from the NRSC). Senate Majority PAC has booked $9.6 million so far; the DSCC did not initially reserve any time in Maine.

Michigan: Gary Peters and John James

Democratic Sen. Gary Peters is running for a second term and is one of just two vulnerable Democratic incumbents. He’s likely to face Republican John James, who lost the state’s 2018 Senate race.

The NRSC has booked $2.7 million so far; Senate Leadership Fund has not yet reserved in Michigan. Democrats have not yet reserved any fall airtime in this state.

Montana: Steve Daines and Steve Bullock

Republican Sen. Steve Daines is running for a second term and will face Democratic Gov. Steve Bullock, who reversed himself to run just before the March filing deadline, turning this state into a battleground.

The DSCC booked $5.2 million for the fall, and the NRSC booked $2.8 million. Neither super PAC has booked for the fall yet.

North Carolina: Thom Tillis and Cal Cunningham

Republican Sen. Thom Tillis is running for a second term and facing Democrat Cal Cunningham, who won a March primary, in what’s likely to be the most expensive Senate race in the country. Both parties have already spent and reserved tens of millions for this race.

Republicans have booked nearly $30 million ($21.8 million from Senate Leadership Fund and $7.3 million from the NRSC). Democrats have booked more than $37 million ($25.6 million from Senate Majority PAC and $11.7 million from the DSCC).

Races that could be competitive and expensive, but where neither party is booked yet: Alabama, Georgia, Georgia Special, Kansas, South Carolina and Texas.