A few months back, as Barack Obama’s campaign was just beginning to look like it might seriously challenge Hillary Clinton’s presidential ambitions, a senior Obama campaign official sent a warning to his staff.

It was a succinct message designed to forewarn and forearm his people against what was expected to be a ferocious onslaught from an increasingly desperate Clinton campaign.

“Better get your hazmat suits on”, the memo told them

If they didn’t do it then, the Obama team should now be reaching urgently for the protective outerwear, because following another night of crushing defeat for Mrs Clinton, it seems that the Clintons have only one remaining hope if they are to wrest the Democratic nomination – a highly toxic attack on the man who is now clearly the frontrunner.

On Tuesday, Mr Obama notched up his ninth and tenth straight victories since Super Tuesday two weeks ago, taking the Wisconsin primary by 15 percentage points – a much bigger margin than expcted – and winning the Hawaii caucuses by 76 per cent to 24 per cent for Mrs Clinton.

The latest triumphs not only cemented the notion that Mr Obama now has the momentum in the Democratic race. It also gave him what looks like an almost insurmountable lead in the so-called pledged delegates to the party’s nominating convention, where the presidential nominee will be chosen.

A rough estimate of those delegate counts shows Mr Obama with a lead of more than 150 out of almost 2,500 chosen so far.

That lead shrinks to just 60 or so when the so-called superdelegates – senior party officials who also get to vote – are added in. But the Obama campaign argues – somewhat persuasively – that if he emerges as the winner in the elected delegate count, it will be very hard for the superdelegates to side with the popular vote loser and they are expected in the end in fact to back the candidate who gets most votes.

Mrs Clinton is running out of primaries to overturn Mr Obama’s lead. The next round of contests in two weeks on March 4 includes two big states that should be favourable to her, Texas and Ohio. But she now not only has to win, but has to triumph by wide margins to remain competitive with Mr Obama.

The most alarming aspect of Tuesday’s defeats for Mrs Clinton was the serious setbacks she experienced, according to exit polls, among key demographic groups that had been her core supporters.

She lost heavily in Wisconsin among white voters and among blue collar, lower-income less educated workers. And she held only the slimmest of margins – a couple of percentage points – among women.

Those groups had been key to her support as she and Mr Obama battled to a standstill. But last week there were signs that the logjam was being broken. In primaries in Virginia and Maryland she appeared to lose ground among her core supporters to Mr Obama.

But Wisconsin’s results were even worse for her than last week’s. In Virginia and Maryland, Mr Obama did especially well because there were large numbers of wealthier voters – who had been a crucial component of his support. But in Wisconsin there were far fewer affluent voters, yet Mr Obama still triumphed, winning comfortably in the working class parts of the state that look very much like the kinds of places that will vote in two weeks time in Ohio and Texas.

Mrs Clinton is running out of options. There’s a growing sense that she may now choose to go aggressively negative. She is under pressure from some in her campaign to step up the attacks on Mr Obama, especially over his inexperience.

She was helped a little last night by John McCain, who won the Republican primary in Wisconsin and virtually wrapped up his party’s nomination. He used his victory speech to pivot to a fierce attack on Mr Obama – suggesting his lack of experience and some of his comments on foreign policy meant he was ill prepared to be commander in chief of the US.

Clinton campaign advisers scrambling to rescue their candidate’s hopes, are indicating that they will echo that line in the next few days. She is expected to argue that Mr Obama is simply hopelessly unprepared to stand up against Mr McCain in the general election campaign.

The Clinton campaign may need to reach deeper into the negative campaigning armoury if they are to have any chance. They will doubtless be looking at Mr Obama’s remarkably low negative ratings in polls and wondering how they can push them up.

Until now the danger has been that negative attacks could backfire. That is a risk that will probably now have to be taken, even if it damages the Democratic party as a whole. The Clintons have only a matter of days to save their lifelong aspirations. They will not give them up without a fight.

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