The only real surprise in the announcement by the Clinton campaign of a senior staff reshuffle was that it took so long.

Patti Solis Doyle, the woman stepping aside as Hillary Clinton's chief of staff, has been the object of repeated internal criticism and intense efforts to unseat her in the past few months. The news that she was being replaced by Maggie Williams, a long-time adviser and friend to the New York senator, was the latest indication of how treacherous and unpredictable Mrs Clinton's once inevitable path to the Democratic nomination has become. It came in the midst of another slew of primary state victories for the surging Barack Obama that are close to providing him with the kind of momentum that, if unchecked, could carry him through to the nomination.

There has been grumbling about Mrs Solis Doyle for most of the past year. It reached a peak in the days after Mrs Clinton's defeat in the Iowa caucuses and before her expected defeat in the New Hampshire primary in January.

As the polls closed in New Hampshire, plans had been laid for a major reshuffle. But as we now know, the expected catastrophe turned into triumph and Mrs Solis Doyle won a reprieve. But since last week's Super Tuesday primaries the Clinton campaign has been under renewed pressure. Though she won comfortably in the big states last week - New York, California, Massachusetts, New Jersey - Mr Obama was able to piece together a patchwork of more numerous victories in smaller states that left the two candidates essentially tied in the race for delegates to the party's nominating convention.

The significance of the reshuffle is that the next two weeks are the point of maximum danger for the Clinton campaign. Mr Obama won the primaries and caucuses in all four states at the weekend. He is expected to win today in the three contests that make up the Potomac Primary - Maryland, Virginia and Washington DC. Next Tuesday he may have an advantage in Wisconsin and should win Hawaii.

That would mean nine straight wins for Mr Obama since Super Tuesday. Mrs Clinton is banking all on winning in the big states to vote next on March 4 - Texas and Ohio. But the risk is that Mr Obama may have established enough momentum by then at least to hold her to a tie.

If he does that, he might have a decisive advantage. Mrs Clinton's principal remaining firewall after that is her support among the super-delegates - the 20 per cent or so of those who will vote at the Democratic convention who are not directly elected by primaries but are largely ex-officio members of the Democratic elite. If they start to sense that Mr Obama is the popular choice they will feel intense pressure to rally to him.

Mrs Clinton, therefore, has to stop Mr Obama from gaining what could be decisive momentum. Clinton campaign advisers insist that this latest move is no sign of panic. Mrs Solis Doyle, they say, had never intended to stay in a full-time managerial role this long (they all thought the campaign would be over by now). But the impact of the decision is clear. With its back to the wall, Team Clinton is signalling that it is ready for a new, critical phase of the campaign.

This will involve an intensification of its efforts to turn out the base to vote for her in the forthcoming primaries. But it will also surely stretch to playing tough as the race for Democratic delegates enters the home straight.

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