Supreme Court Chief Justice John Roberts reportedly initially sided with conservatives on the court to strike down the heart of the health care law - but later changed his mind.
CBS News reports that Roberts formed an alliance with liberals to uphold most of the law, a move that drew the ire of his fellow conservative justices.
Justice Anthony Kennedy launched a concerted effort to sway Roberts back to his original posItion.
"He (Kennedy) was relentless," one source told CBS News. "He was very engaged in this."
No one is certain why Roberts switched sides. Some analysts have speculated he was keenly aware of pressure from the media and liberal politicians.
"The fact that this decision was apparently political, rather than legal, completely undermines its legitimacy as a precedent," Randy Barnett, a professor at Georgetown Law, said in a statement.
"Its result can be reversed by the people in November," he predicted. "And its weak tax power holding reversed by any future court without pause."