"You sort of get whipsawed," Cahill said.
A year ago, he was tied in a statewide Boston Globe survey with Democratic Gov. Deval Patrick in a three-way race, and he was on par with or ahead of Republican Charlie Baker in most statewide polls until this spring.
Then the Republican Governors Association unleashed more than $1 million in radio and TV ads attacking Cahill as "just another Beacon Hill politician" who was "practically the same" as Patrick.
Republicans "need him out of this race because he's dividing the anti-Patrick vote," said Jennifer Duffy of the non-partisan Cook Political Report.
The negative barrage succeeded in pushing down Cahill's support to 9 percent in a Boston Globe-University of New Hampshire poll in June.
"It has become more difficult," Cahill acknowledged in an interview. "I had a very formidable positive-negative rating, and they were successful in turning it upside down; now it's a net negative." He had hoped to save his resources until the fall, but he launched his first TV ads last week to try to rebound.
"My message," Cahill said, "was resonating with people."