When he took the stage at campaign rallies across the country, presidential hopeful Donald Trump often had a piece of paper in his pocket, ready to go if the moment seemed right.

On it was printed something unlikely: the lyrics to a song written in the 1960s by civil rights activist Oscar Brown Jr. Based on a fable, The Snake tells the story of a kind woman who takes an injured snake into her home to nurse him back to health only to be shocked when she is fatally bitten.

In Trumps hands, however, Browns words were imbued with a new and specific meaning.

As the opening scene from the upcoming FRONTLINE documentary Zero Tolerance explores, Trump recast the lyrics as an anti-immigration allegory equating people seeking to enter America with killer snakes who would inevitably cause harm to those who welcomed them. Trump built to a crescendo around the songs last lines, an exchange between the dying woman and the snake:

I saved you, I saved you, I saved you, cried that woman.And youve bit me, heavens why?You know your bite is poisonous and now Im going to die.Oh shut up, silly woman, said the reptile with a grin.You knew damn well I was a snake before you took me in.

Trumps embrace of that comparison kicks off Zero Tolerance, a FRONTLINE documentary premiering Tuesday, Oct. 22, that investigates how the president turned anti-immigration fervor into a powerful political weapon. The films opening moments depict Trumps use of The Snake in such a way that its never before been seen juxtaposing the then-candidates reading with stark images of men, women and children struggling to reach the U.S.-Mexico border.

As the documentary explores, Trumps comparison of immigrants to killer snakes was just the beginning. He would go on to make opposition to immigration the signature policy of his presidency, using virulent anti-immigration sentiment to animate his supporters and fuel a political movement. He continued to recite the lyrics to The Snake at several public events as president.

He tapped into something in a very profound way that began to redefine the debate in the political year of 2016 and continues to redefine the politics of the country today, Dan Balz of The Washington Post tells FRONTLINE in the above clip.

Trump didnt do it alone. The documentary, from veteran FRONTLINE filmmaker Michael Kirk and his team, tells the inside story of how an improbable group of outsiders Stephen Miller, Jeff Sessions and Steve Bannon became the driving force behind Trumps most controversial initiatives, as part of a grand anti-immigration strategy they would call Zero Tolerance.

Miller is the only one still left in the Trump administration, but the film shows how the trios years of planning around immigration are still shaping decisions. Zero Tolerance is a must-watch look at how Americas immigration policies and the rhetoric surrounding them reached this moment.

Zero Tolerancepremieres Tuesday, Oct. 22 at 10 p.m. EST/9 p.m. CST. Tune in or stream onPBS (check local listings), at PBS.org/frontline and on thePBS Video App. More than 30 interviews from the making of the documentary will go live on FRONTLINEs website in tandem with the premiere, in the newest installment of The FRONTLINE Transparency Project.

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WATCH: Donald Trump and The Snake | Zero Tolerance | FRONTLINE - FRONTLINE