Key Points:

NEW YORK - Hillary Clinton was facing fresh questions yesterday over her private political and financial records after revelations that the presidential library of her husband is blocking the release of more than 1,000 pages of documents pertaining to pardons doled out in the twilight of his presidency.

After losing his momentum last week in the slog for the Democratic nomination, Barack Obama is preparing to pay Mrs Clinton back for her negative attacks on him, particularly by highlighting her reluctance to release these and other records, including the family tax returns.

The Obama campaign manager, David Plouffe, excoriated the former first lady in a conference call with reporters, labelling her "one of the most secretive politicians in America today".

While Mr Obama insisted he would not be drawn "into a knife fight" with Mrs Clinton, his campaign has come under pressure from supporters to be more robust in countering her attacks on him.

A Newsweek poll yesterday confirmed that while Mrs Clinton's victories last Tuesday in Ohio, Texas and Rhode Island still left her trailing substantially in the scramble for nominating delegates, she is nonetheless gaining ground psychologically.

It showed her eroding her opponent's recent national lead, attracting the support of 44 per cent of Democrat voters against 45 per cent for Mr Obama.

A much-needed fillip was potentially in the offing for Mr Obama, however, as Democrats in Wyoming showed up in record numbers to vote in caucuses, which usually bodes well for him.

Both candidates, as well as Bill Clinton, were in the Cowboy State late last week, trying to wrangle votes.

The state is the least populated in the US and is heavily Republican, but at this stage, no one can afford to ignore even this windblown state, where a paltry 12 delegates are at stake.

Voters in Mississippi, which has its primary on Tuesday, are even more likely to fall into the Obama column.

Further ahead, however, he faces an uphill climb in Pennsylvania, the next big battleground, on 22 April.

A loss there will bolster the argument that he has difficulty capturing the big states with big working-class populations that will be crucial to victory in November.

Some of the pardons extended by Mr Clinton just before vacating the White House were deeply controversial, notably for the fugitive financier Marc Rich, whose wife, Denise Rich, was a family friend and huge contributor to the party and to Clinton campaigns.

Much more dangerous, potentially, are two other sets of papers - the Clinton tax filings and records of Mrs Clinton's activities while First Lady.

Mr Obama has been pressing heavily for the release of both sets of documents, contrasting her claims of superior experience because of her White House years with her unwillingness to have these papers scrutinised.

Mrs Clinton has indicated that the financial filings would be released on or around 15 April.

Particular scrutiny will be paid to the sources of the Clinton family fortune - some of which has been tapped to fund her campaign - and of recent donations to the Clinton library.

"Are there favours attached to $500,000 or $1m contributions?" asked Bill Bradley, a former senator and an Obama supporter.

"And what do I mean by favours? I mean, pardons that are granted; investigations that are squelched; contracts that are awarded; regulations that are delayed."

Campaigning in Mississippi, Mrs Clinton raised for the second time the possibility of a Democratic "dream ticket".

"I've had people say, 'I wish I could vote for both of you'. Well, that might be possible some day," she remarked in Hattiesburg.

Mr Obama has said that any such talk is "premature".

If there is any joining of the two, it may for now only be in the gutter.