"Calls and emails are more than 500-to-1 against this bill to Governor Haslam. There is no reason for him to sign it," said HSUS President and CEO Wayne Pacelle.
Pacelle is in Nashville for the organization's annual Animal Care Expo, held at Gaylord Opryland Resort & Convention Center.
The four-day education conference and trade show draws an estimated 1900 people from 40 countries.
An HSUS news conference is scheduled for Thursday to discuss soring in the Tennessee Walking Horse industry.
"This practice of injuring the feet or the hooves of horses to prompt them to have so much pain that they exaggerate their gate and they win ribbons at horse shows," Pacelle said. "This abuse of horses has become routine in the industry. It's terrible to think that it's become this bad, but the evidence is incontrovertible."
An HSUS-lead undercover investigation shocked the nation and rocked the industry last year. Hidden cameras captured prized equines being beaten and sored by horse trainer Jackie McConnell.
"This Ag-Gag bill is an attempt to cover-up abuse. They don't want us to be able to get into stables or factory farms to see terrible abuses of animals," Pacelle said. "The only people who have to worry about this are the people who are violating the law or people who are doing terrible things to animals."
The HSUS is among countless animal rights groups and celebrities, including Carrie Underwood and Ellen Degeneres, who have decried the bill.
However, supporters have long argued the bill is intended to encourage whistleblowers to come forward and stop animal abuse as quickly as possible.
Governor Haslam continues to weigh both sides. He's also looking to State Attorney General Robert Cooper for an opinion on whether the bill is constitutional.
The deadline for the governor's signature is May 15.