MIAMI — As Marco Rubio debated whether to run for the United States Senate in 2010, he made a pledge steeped in loyalty and deference: He would never challenge his political mentor, Jeb Bush, if the former Florida governor wanted the job himself.

This time around, Mr. Rubio is not asking for permission.

Mr. Rubio’s plan to enter the presidential campaign here on Monday, upstaging Mr. Bush in his own backyard, signals a decisive, Shakespearean turn in a 15-year relationship so close, personal and enduring that friends describe the two men as almost family.

It is a bond that has stretched from Miami to Washington, punctuated by late-night telephone consultations, fueled by mutual enemies and fixated on reinvigorating conservatism with big ideas, according to dozens of interviews.

The head-on collision of Mr. Bush’s and Mr. Rubio’s presidential aspirations, once viewed as an outlandish prospect in this state, is now consuming their hometown with talk of unchecked ambition and political backstabbing.