IT HAS been another tough week for Barack Obama, ending a cruel month that he must be glad to see the back of. This Tuesday's primaries in Indiana and North Carolina offer an immediate opportunity to move on from his campaign's first real crisis. His popular support has eroded and his energy is waning, but anything other than a shock double loss will speed his long, painful, limp to the finish line.

April put paid to the inevitability argument once and for all. It had already served Hillary Clinton poorly, creating a backlash against her ties to the party establishment. From Iowa to Virginia, Democrats bridled at the suggestion that she was the only candidate with the experience or the connections to win the nomination. Unfortunately for Obama, during his string of victories in February he adopted the same air of certainty, so easily taken for arrogance.

He has struggled desperately ever since, against a relentless, revitalised Clinton. Obama can blame the hard questions that come with being a front-runner, the media's interest in prolonging the race, the focus on "pastor disaster" Jeremiah Wright, or simply a sequence of states where the demographics favoured his opponent. But the timing is telling - he stumbled the moment his campaign said it was impossible for Clinton to catch him.

Even Clinton's most ardent fans recognise that she has no chance of overturning Obama's lead in pledged delegates. They know that claiming to have won more votes overall will probably require some creative accountancy with the Florida and Michigan primary results.

But until Obama settles it with a decisive victory, they believe that the only jury that counts is still out. There are more than enough uncommitted superdelegates to push either candidate across the tape.

Indiana and North Carolina matter most as a fresh set of data to spin, to bolster the electability argument that is Clinton's only hope. Although neither are swing states, they offer a series of clues to how the candidates fare with key voting blocs, chief among them, once again, white men without a college degree. They will also provide the first conclusive evidence of how badly the re-emergence of Reverend Wright has damaged Obama, both in the short term and as a potential general election candidate.

It has long been assumed that Obama will sweep North Carolina. More than a third of the primary electorate is African-American. If he again wins an astounding 90% of the black vote, as he has done fairly consistently in recent primaries, Clinton will need three-quarters of the state's white voters to beat him. Incomes are higher and university education more widespread than in Pennsylvania, another factor that favours Obama.

His lead has narrowed though, from an impregnable position to slim single digits in the space of a fortnight. Governor Mike Easley's endorsement should help Clinton keep the margin tight, thanks to his standing with blue collar workers and gun owners. Easley is the two term Democrat leader of a staunchly Republican state. He is Catholic, socially conservative, a keen hunter and a strong advocate of the Second Amendment right to bear arms. Although he has not said so explicitly, his aides believe Clinton can win.

North Carolina is the kind of upset victory that Clinton needs, to prove she can shatter Obama's coalition whilst maintaining a lock on her own base. A win in Indiana, where she leads most recent polls, would mean less, even if it is as convincing as Pennsylvania. The maths is dull from repetition, but at this point, a draw can only help Obama.

Indiana is a Republican banker, red since Richard Nixon. The last Democrat to take its electoral college votes was Lyndon B Johnson in 1964. Its people, who call themselves Hoosiers, tend to care about wedge issues such as gay marriage, abortion and gun control. They favour small government and low taxes, cheer for college basketball teams and watch Nascar races at the weekend.

The few Democrats that have succeeded here are from the fiscally conservative wing of the party. Senator Evan Bayh is one of Clinton's most loyal lieutenants. Congressman Baron Hill, from the southern district that includes Clarksville and Bloomington, endorsed Obama last week.

As a rust belt state next door to Ohio, historically inhospitable to liberals, Indiana is a good fit for Clinton, whose stump speech about "jobs, jobs, jobs, jobs" resonates in steel industry towns that have suffered through wave upon wave of redundancies. But it is also a neighbour to Illinois, meaning that voters in the northwest get Chicago television. Obama has yet to lose a state that borders his home territory and has already won Iowa, Missouri and Wisconsin.

Obama usually does well in Republican states where the Democratic party apparatus is weak, thanks to his young, well-organised volunteer operation. He also attracts more independent voters than Clinton, so Indiana's open primary is good news. Until Reverend Wright stepped back into the spotlight, the state looked like a toss-up.

Wright's defiant performance at the National Press Club threw a hand grenade into Obama's lap. He began by scolding everyone who saw two short video clips and rushed to judgement, but soon revealed a man every bit as angry and as self-regarding as his YouTube caricature.

"I do what pastors do, he does what politicians do," was Wright's supreme understatement, as he ran down a list of radical viewpoints, alternating paranoid delusion with unpalatable truths that candidates must avoid like quicksand. He observed that American foreign policy invites terrorist attack, compared US marines to the Roman soldiers who crucified Jesus and repeated his accusation that the HIV virus was created to kill black Americans. "Our government is capable of anything," he concluded.

Obama inexplicably took a day to respond. When he did, the nuances of his celebrated speech on race were conspicuously absent. "I'm outraged by the comments that were made and saddened by the spectacle that we saw yesterday," he said. "They offend me, they rightfully offend all Americans, and they should be denounced." Clinton has so far refrained from putting the boot in. An advert calling Wright and Obama "too extreme for North Carolina" was paid for by local Republicans.

When Obama ended his boycott of Fox News, Clinton followed suit. Her first question, served with a smile by right-wing presenter Bill O'Reilly, was: "Can you believe this Reverend Wright guy?"

Democrats don't often campaign in Indiana, because the nomination is wrapped up by May and the general election result is a foregone conclusion. This year, at rallies in Marion, Terre Haute, Gary and Columbus the candidates have spelled out their plans to revive manufacturing industry. They have traded sporting metaphors and everyman anecdotes. Both use a John Mellencamp anthem, Our Country, as exit music.

Obama played pick-up basketball for the cameras. Clinton commuted to work with a panel beater in his white Ford truck. The South Bend Tribune reported that "for half a tank of regular unleaded gasoline, they paid $63.67 £32.27."

Petrol taxes have emerged as one of the few areas of policy that Clinton and Obama flatly disagree about. Both John McCain and Clinton have proposed a "gas tax holiday" for the summer, during which the government levy of 18.4 cents per gallon would be suspended. The only significant difference between their plans is that Clinton would impose a windfall profits tax on oil companies to pay for it.

Obama presents this as political pandering. In North Carolina he called it a false economy that would save the average family $28 while putting public sector construction jobs in jeopardy. "This is the problem with Washington," he said. "Oil companies like Shell and BP just reported record profits for the quarter. And we're arguing over a gimmick that would save you half a tank of gas over the course of the entire summer so that everyone in Washington can pat themselves on the back and say that they did something."

New York Times columnist Thomas Friedman described it as money laundering: "We borrow money from China and ship it to Saudi Arabia and take a little cut for ourselves as it goes through our gas tanks." Huffington Post writer Sam Stein tried to assess independent support for the plan, but couldn't find a single economist or energy expert who would back it.

Robert Shapiro, a former undersecretary of commerce in Bill Clinton's administration, called it "trivial or worse - by reducing the price of driving it encourages more of it, thereby increasing demand for gasoline, which inevitably pushes the price back up - the consumer gains nothing, and the oil companies and OPEC collect the extra bucks instead of the government."

None of which has deterred Clinton. "My opponent, Senator Obama, opposes giving consumers a break," she told a town hall meeting in Indiana. "I understand the American people need some relief." Her calculation, that voters choose self-interest over the greater good, has been proved right before.

If Clinton can somehow win both contests on Tuesday and ride that momentum into West Virginia, Kentucky and Oregon, it will support her claim that Obama is too liberal to defeat McCain. She has already compared him to Al Gore and John Kerry, suggesting that they lost because "large segments of the electorate concluded that they did not really understand or relate to or frankly respect their ways of life."

From there, it is a short leap to the definitive Democrat loser of the modern era, Senator George McGovern. Writing in the New Republic, John Judis argued that an examination of Obama's support in Pennsylvania revealed "the outlines of the old McGovern coalition that haunted the Democrats during the 70s and 80s." It is the perfect spectre to scare waverers.

Clinton's problem is that during the first sustained rough patch of Obama's campaign, the flow of superdelegates is still in his direction, not hers. He has halved her lead in the past two months, to around 20. Both campaigns make a fuss each time they secure a previously uncommitted super, but the big defections are always from Clinton to Obama.

This week, just when he needed it most, two more of Clinton's prominent supporters switched sides. On Thursday, Joe Andrew, who led the Democratic National Committee during Bill Clinton's presidency, announced that he was transferring his allegiance to Obama.

At a press conference in his home town of Indianapolis, he told reporters "this has got to come to an end a vote for Hillary Clinton is a vote to continue this process and a vote to continue this process is a vote that assists John McCain." The next day Paul Kirk, another former DNC chairman, followed his example. Unless Clinton can reverse the tide, she will be swept away in June.