Back in September 2005, a certain neophyte Democratic Senator voted 'No' to the confirmation of John Roberts as Chief Justice of the United States. Now Barack Obama should be sending Mr Roberts a grovelling letter of apology confessing his error.

With his decisive fifth vote to uphold the key provision of the Affordable Care Act (ACA) – aka "Obamacare" – a Chief Justice appointed by a Republican president has handed a Democratic president his most important victory. He may also have saved the reputation of the Supreme Court itself.

A decision to gut the bill of the "individual mandate", or to strike down the healthcare reform measure in its entirety, would have been a simple 5-4 victory for the court's conservative majority. Such an outcome was the general expectation beforehand, after a string of similar narrow wins on politically charged cases.

In those confirmation hearings seven years ago, Mr Roberts declared, employing a baseball metaphor, that his job was "to call balls and strikes." Reality has been different. At the crunch, the supposedly impartial Court, the third branch of the constitution, has all too often seemed part of the GOP, aggressively pushing conservative doctrine. Probably as a result, public confidence has ebbed. Normally one of the country's most trusted institutions, the Court's popularity has lately slipped to barely 40 per cent, the lowest in decades.

This week the tide may have started to turn. On Arizona's hardline immigration law, the Court had something for both sides. It struck down three provisions, but on the most controversial – allowing police to demand the papers of suspected illegal immigrants – it took the line of "let's see how it works" before taking a final decision.

Yesterday's healthcare ruling was in similarly restrained vein. The "individual mandate" (obliging Americans to buy health insurance if they don't have it,) could be construed as a tax, Chief Justice Roberts wrote, as he unexpectedly sided with the four liberal justices. "Because the Constitution permits such a tax, it is not our role to forbid it," he said. Nor was it for the Court to pronounce upon the basic wisdom of the Act; that was up to "the People," – the politicians and Congress. Exactly how it should be.

By upholding the ACA, the Court has not ended the healthcare reform argument. Most Americans opposes Obamacare. More than ever, the issue will feature in the election campaign; more than ever Republicans will step up their efforts to repeal the Act. The ruling may help them, by galvanising activists and relieving the party of the need to say what it would put in the ACA's place.

Moreover, by deeming the mandate as a tax, the Court has backed Republican claims the ACA is a tax increase. Expect to hear that refrain daily on the campaign trail. If Mitt Romney wins in November, and the Republicans take full control of Congress, then Obamacare will be gone. That is the right of a party that wins elections. It is not the right of a Supreme Court that has a pro-Republican majority. John Roberts recognised that fact.