By Alison Gendar

WASHINGTON — It wasn’t just President Obama’s legacy on the line Thursday — so was the chief justice’s.

John Roberts feared the highest court’s reputation would be sullied if politics was seen to be the deciding factor on Obamacare, experts said.

The Harvard-educated justice’s stance Thursday showed even on hot-button issues “he can put aside whatever his personal politics might be and call balls and strikes,” court-watcher Elizabeth Wydra said.

Roberts, 57, famously claimed during his 2005 confirmation hearing that he’d act as an umpire from behind the bench — not an activist.

A then-Sen. Obama didn’t buy it, and voted to spike Roberts’ nomination.

But seven years later, Roberts held true to his word — and saved Obama’s defining legislative action.

Roberts rose from rural Indiana to the top job on the bench, marrying fellow power lawyer Jane Sullivan along the way. The couple has two adopted children, Jack and Josie.

Roberts cut his teeth at the Supreme Court as a clerk for late conservative Chief Justice William Rehnquist.

Nominated by President George W. Bush, he was not considered to be the likely swing vote.

The chief justice, though, said in the ruling that his job isn’t to judge the quality of the health care legislation.

“Members of this court are vested with the authority to interpret the law; we possess neither the expertise nor the prerogative to make policy judgments,” Roberts wrote. “It is not our job to protect the people from the consequences of their political choices.”