*Editor's note: This story has been updated throughout.
"They just desperately want somebody new," Fallon said of voters in Senate District 30, which Estes has represented since 2001. "It's been 16 years — it's going to be 18 years. They want a change. They don't see him around."
Fallon had been seriously mulling a Senate bid for months, crisscrossing the 14-county district in North Texas since at least the end of the regular legislative session in May. He first shared his decision to run Tuesday with a newspaper in SD-30, the Weatherford Democrat.
In an interview with the Tribune, Fallon said he was "shocked" to learn in his travels how many local officials view Estes as an absentee senator. Fallon, who loaned his campaign $1.8 million in June, also said he was prepared to "spend every dime and then some" to get his message out in the race.
"It's a moral obligation," he said. "We simply need in this district to close one chapter and open up a new one."
Fallon said he already has the backing of 25 GOP members of the House, including four in the area of SD-30: Mike Lang of Granbury, Ron Simmons of Carrollton, Jeff Leach of Plano and Scott Sanford of McKinney.
Estes is the second-longest serving Republican in the Texas Senate, where he chairs the Natural Resources & Economic Development Committee. Estes' campaign did not immediately respond to a request for comment late Tuesday night, but he announced in June that he was seeking another term.
"I plan to run a positive grassroots campaign across the entire district based on my proven record of conservative leadership," Estes said at the time.
The incumbent's campaign announced in June that he had the support of five GOP state representatives from across SD-30. Two of them, Tan Parker of Flower Mound Lynn Stucky of Denton, said Wednesday they have since decided to stay neutral in the race after learning of Fallon's candidacy.
"While historically I have endorsed Senator Estes' campaign, this recent development of having two colleagues from my area running for the same seat has led me to instead take a neutral position," Parker said.
Fallon is serving his third term representing House District 106 in Denton County. Two Republicans, Clint Bedsole and Jared Patterson, have already lined up to run for the seat in 2018.