The governor's veto infuriated religious conservatives who considered the measure, House Bill 757, their top priority. This is the third legislative session they've sought to strengthen legal protections from opponents of gay marriage, but last year's Supreme Court ruling legalizing same-sex weddings galvanized their efforts.
It is also likely to herald a more acrimonious relationship between Deal, who campaigned on a pro-business platform, and the evangelical wing of the Georgia Republican party. Already, prominent conservatives have vowed to revive the measure next year.
More: How Deal's 'religious liberty' veto will shape his final years in office
The governor, though, had ample cover from the measure's critics. Executives from dozens of big-name companies, including Disney, Apple, Time Warner, Intel and Salesforce, called on the governor to veto the bill. The NFL warned it could risk Atlanta's bid for the Super Bowl and the NCAA hinted it could influence the state's ability to host championship games. And Deal's office said two economic development prospects have already abandoned Georgia because of the legislation.
They joined with gay rights groups who warned that the measure amounts to legalized discrimination and pointed to the corporate outrage that rocked Indiana after a similar measure was signed into law there.
The legislation, which first surfaced on March 16 and passed both Republican-controlled chambers in hours, would allow faith-based organizations to deny services to those who violate their "sincerely held religious belief" and preserve their right to fire employees who aren't in accord with those beliefs.
It also mirrors language found in the federal Religious Freedom Restoration Act, which was signed by President Bill Clinton and adopted by dozens of states, requiring government to prove a “compelling governmental interest” before it interferes with a person’s exercise of religion. And it includes a clause saying it could not be used to allow discrimination banned by state or federal law.
Seen by supporters as a "compromise" effort, the measure was swiftly condemned by the Metro Atlanta Chamber, the state's most influential business group, and by leaders of major international tech corporations.
The Human Rights Campaign called on Hollywood film companies to abandon Georgia if Deal signs the measure, and many issued threats that they would. Each of Atlanta’s pro sports franchises criticized the measure, as did the owner of the Atlanta Falcons.
It was far from a one-sided fight, however. The conservative Faith and Freedom Coalition launched robo-calls backing the measure, and the Georgia Baptist Mission Board marshaled its 1.3 million members to rally around the bill. State Sen. Josh McKoon and other prominent supporters cast it as a way to protect faith-based beliefs.
"I'm extremely disappointed," McKoon, R-Columbus, told WBUR Boston shortly after the veto. "This bill had been significantly watered down. It did not apply to businesses. I'm just very, very disappointed the governor would veto this modest protection for people of faith."
Still, Deal’s decision to veto the measure did not come as a surprise.
In stark terms, the governor said earlier this year that he would reject any measure that "allows discrimination in our state in order to protect people of faith." Rooting his critique in biblical language, he urged fellow Republicans to take a deep breath and "recognize that the world is changing around us."
He is also the rare statewide politician who can afford to infuriate a wide swath of his party’s base. As a term-limited governor with no further political ambitions, he never has to face the voters again.
Yet his decision will likely influence the remainder of his term, which ends in January 2019.
The "religious liberty" debate resonates like few others among the activists that make up the Georgia Republican base – a group that gave the legislation a ringing endorsement at the Georgia GOP's 2015 convention. He'll need many of those same rank-and-file Republicans next year when he unveils his plan to "revolutionize" the state's education system.
Already, several conservative lawmakers have vowed to call for a "veto session" to rebuke the governor if he rejects the measure. It takes a three-fifth majority in both chambers to call a special session, and a two-thirds majority in both chambers to override a veto — a threshold the bill failed to reach by one vote in the Senate and 16 in the House.
State Sen. Bill Heath, one of the chamber's most conservative lawmakers, said he's confident a "veto session" will be successful.
"We will call for a veto session," Heath said. "And we have the votes."
The governor, who didn't take any questions after his remarks, anticipated the pushback.
"I don't respond well to insults or threats," he said.
Insider's note: Aaron Gould Sheinin and Kristina Torres contributed to this report.