Republicans have offensive targets in Alabama and Michigan, two seats that could help them preserve their control of the chamber even if they lose seats elsewhere. But Democrats have also recruited strong challengers in a string of red states that could come into play, including Montana, Kansas and a special election in Georgia, helping them potentially expand their path back to the majority.

J.B. Poersch, president of Senate Majority PAC, Democrats’ top Senate-focused super PAC, said the Democratic presidential primary, Trump’s impeachment trial and the coronavirus outbreak have created a roller coaster political environment in the past 18 months. But despite that, their targets have been stable.

“There seems to be common thinking in both parties that this is a map, and perhaps an environment, that favors Democrats,” Poersch said. “There's been a consistency in the year-and-a-half of Democrats performing well in many of these battleground states, so we're encouraged.”

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 with $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.