Maine, one of the smallest battlegrounds, has emerged as the most expensive so far. Sen. Susan Collins faces the most competitive election of her career: Democrat Sara Gideon raised more than $7 million in the first quarter of this year, compared to $2.4 million for Collins. The two candidates alone spent more than $7 million in the first quarter,

Gideon has also been boosted by pro-Democratic outside groups — including two nonprofit, “dark money” groups — which have outspent Republicans by a 2-1 margin.

Arizona and Colorado remain Democrats’ best targets, thanks to star recruits. In Arizona, former astronaut Mark Kelly is the best funded candidate in the country, sitting on $19.7 million, which is nearly double the cash of Republican Sen. Martha McSally. And former Colorado Gov. John Hickenlooper, who has won statewide twice, outraised Sen. Cory Gardner in the first quarter of this year, though Gardner still holds a large cash advantage and Hickenlooper faces a June primary.

North Carolina has emerged as the possible tipping point race for the majority. Democrats invested $13 million to boost Cal Cunningham through his March primary, while Republicans spent $2.5 million aiming to damage Cunningham by boosting his more liberal opponent, whom he defeated handily.