HOW can Americans overcome their divisions? Barack Obama, the son of a lapsed Kenyan Muslim, has some arresting thoughts. On the subject of tackling head-on “the mutual suspicion that sometimes exists between religious America and secular America”, the junior senator from Illinois delivered last week one of the best speeches of his brief career.

He told the story of a doctor who wrote to him when he was running for the Senate in 2004. The doctor said he might vote for Mr Obama, but was repelled by a line on his campaign website promising to fight “right-wing ideologues who want to take away a woman's right to choose”. The doctor wrote: “I sense that you have a strong sense of justice, [but] whatever your convictions, if you truly believe that those who oppose abortion are all ideologues driven by perverse desires to inflict suffering on women, then you, in my judgment, are not fair-minded.”

Mr Obama says he “felt a pang of shame”. The offending words, which he called “standard Democratic boilerplate language”, had been posted on his website by campaign staffers. He had them changed; not because he had changed his mind about abortion, but because he wanted to “extend the same presumption of good faith to others that the doctor had extended to me”.

Concerning the proper role of religion in politics, Mr Obama cautions against extremism of both stripes. Believers cannot abandon what they believe; but in a nation that includes Christians, Jews, Hindus, Muslims, Buddhists and non-believers, “democracy demands that the religiously motivated translate their concerns into universal, rather than religion-specific, values.” Even if all Americans were Christian, it would not be easy to decide which passages of scripture should guide public policy. “Should we go with Leviticus, which suggests slavery is OK and that eating shellfish is abomination?” he asks. “Or should we just stick to the Sermon on the Mount—a passage that is so radical that it's doubtful that our own Defence Department would survive its application?” His elegantly non-committal answer: “Before we get carried away, let's read our Bibles.”

For someone so inexperienced, and whose policies are so ill-defined, Mr Obama is extraordinarily popular. He is only 44, but people are already begging him to run for president. Something about him fills a gap in American politics: he seems not to be faking when he talks of mending America's religious and racial divides. He is that rare thing, a black politician who addresses the whole nation, not just an ethnic enclave.

That this is rare is tragic. It is also virtually inevitable, given the way the electoral system works. As a senator, Mr Obama is accountable to an entire state's voters. But every other black member of Congress sits in the House of Representatives, where most represent gerrymandered majority-black districts. Unlike Mr Obama, they need not bother appealing to whites. They need not worry about the ideological centre ground, either; since no Republican can win a majority-black district, the crucial contest is the Democratic primary, in which only the most passionate Democrats vote.

Racial gerrymandering has two effects. First, and most conspicuously, it allows some crummy candidates to win by prodding racial sore spots. Cynthia McKinney, for example, a congresswoman from Georgia, seems to believe that every misfortune that befalls her or America is somehow rooted in racism. When she was reproached for punching a policeman in March, the real issue, she said, was that he was a racist for not recognising her. Had he realised her rank, he would not have stopped her as she strode past him. Ms McKinney is also known for her interest in conspiracy theories about the murders of Martin Luther King and the rapper Tupac Shakur, and about President George Bush's supposed foreknowledge of the attacks of September 11th 2001. That last enthusiasm cost her her seat in 2002, a misfortune her father blamed on the Jews. But she won it back in 2004.

The second effect of racial gerrymandering is less obvious, but more important. Most members of the congressional black caucus are fine politicians. But the process by which they are chosen practically guarantees that they cluster near one pole of American politics, to the left of most Democrats and indeed most blacks. This makes them less influential than they should be, even when Democrats control the House. And they find the House a rotten launching pad for higher office, because running in a 60% black district is poor preparation for a statewide campaign. Mr Obama is the only black senator, and there are no black governors.

Why polarisers prevail

Fans of racial gerrymandering argue that the Voting Rights Act of 1965 requires it. This is not obvious from the text, but bureaucrats and judges have read it that way. Fretting that racially separate voting districts depend on the “demeaning assumption that voters of a particular race...think alike,” the Supreme Court has occasionally struck down the most contorted gerrymanders (including, in 1995, the first district in Georgia to elect Ms McKinney). But most pass muster, and the civil-rights establishment is zealous in their defence. There was a big hoo-hah two weeks ago when House Republicans postponed a vote to renew for another 25 years certain emergency provisions of the Voting Rights Act—provisions that had originally been due to expire in 1970. This was wrongly portrayed as a reluctance to renew the act itself, which is permanent.

The right of black Americans to vote is no longer up for debate. Unfortunately, there is not much debate about gerrymandering either. Incumbents like picking their voters, whether they are black, white, Republican or Democratic. The practice may help polarise America along racial, religious and political lines, but it also helps them keep their jobs.