US Supreme Court deals blow to tough Arizona immigration law

The US Supreme Court kicked off its most momentous week in a decade yesterday by overturning large parts of a controversial Arizona law cracking down on illegal immigration.

In another high-profile case, it also served notice that it would not retreat from a heavily criticised 2010 ruling that opened the floodgates to unlimited corporate and private donations for political campaigns.

Three of the four contested provisions in the Arizona law, which was passed two years ago, were thrown out. But a clause bitterly opposed by Hispanic and other immigrant groups, that requires police to check the papers of a person they suspect is in the country illegally, was kept in place – for the time being at least.

On balance, the outcome was a setback for the anti-immigration lobby, and could hit efforts by other states, including Alabama, South Carolina and Georgia, to clamp down on illegal immigrants. The move also made it more certain than ever that immigration will be a major issue in the presidential campaign.

However, the court's decision on campaign finance is another blow to efforts to limit the flow of private money into politics in an election year where campaign spending could reach an unprecedented $6bn. The challenge had come from Montana, whose state legislature defied the Supreme Court's 2010 Citizens United ruling, arguing that Montana politics were especially vulnerable to corruption by unchecked corporate money.

That law was upheld by Montana's state supreme court, but the highest US court struck down it yesterday, with the five conservative justices outvoting the four liberals on the bench.

Citizens United has reshaped US campaign financing by allowing corporations, unions and rich individuals to pour vast sums into the presidential and other races through super political action committees (super-Pacs), which can raise and spend unlimited sums. In theory, they are barred from co-ordinating their efforts with the candidates they support, but that ban is honoured only in the breach.

All eyes are now on Thursday, when the court will rule on whether President Obama's 2010 healthcare law is constitutional. It is the court's most politically charged case since the Bush vs Gore ruling that handed the White House to George W Bush in December 2000. On that occasion the court's conservative majority prevailed by 5-4, and a similar outcome, for all or part of "Obamacare", is on the cards now. Although judges may not throw out the law in its entirety, legal experts believe they may well strike down the central provision of the "individual mandate", requiring Americans without health insurance to buy it. That would be a demoralising defeat for Mr Obama, largely nullifying the legislative achievement he hopes will be his legacy. The implications for the 2012 election are less clear, however. Republicans will claim victory but Mr Obama's supporters could also be energised, directing their fire on a supposedly neutral Supreme Court that, they say, in practice is the Republican party dressed in legal robes.