Daines raised approximately $415,000 in the last three months, according to a source close to the campaign who said it "demonstrates Daines's growing consideration of this race."

That's a jump from the $250,000 he raised in the first three months of the year.

The first-term congressman is seriously mulling a run for the seat held by retiring Sen. Max BaucusMax BaucusChanging of the guard at DC’s top lobby firm GOP hasn’t reached out to centrist Dem senators Five reasons why Tillerson is likely to get through MORE (D-Mont.), and is the GOP's most likely top-tier candidate. Democrats are hoping that former Montana Gov. Brian Schweitzer (D) will jump in on their side. Recent polls show a tight race if the two face off.