Hillary Clinton sought to skewer Barack Obama on the eve of critical voting in Ohio and Texas, pointing to a leaked memo purportedly showing that his campaign recently gave a "wink-wink" to the government of Canada that promises he has made to renegotiate trade deals need not be taken seriously.
With polls showing them in a dead heat in both states, Mrs Clinton has been ratcheting up her attacks on Mr Obama on issues ranging from free trade, health care, the economy and national security. Today may be do-or-die for the former first lady as she tries to end Mr Obama's 11-state winning streak.
Both campaigns have been scrapping furiously over Nafta, the free-trade pact between the US, Canada and Mexico, which for many Ohioans has become a hated symbol of job losses and economic decline. Both have vowed to revisit the treaty and any notion of insincerity by Mr Obama could hurt him.
Today's votes could turn out to be climactic. By most reckonings, Mrs Clinton needs to hold on to both Ohio and Texas to stay viable in the nomination race. As the double-digit leads she held in the states as recently as mid-February have melted away, she finds herself fighting for her political life.
For John McCain, today's voting across four states – Vermont and Rhode Island also go to the polls – should further reinforce his status as the presumptive Republican nominee and bring him within a hair's breadth of the 1,191 delegates that he needs to sew it up and send his last remaining rival, Mike Huckabee, packing.
The trade imbroglio centres on a Canadian embassy memo leaked to the Associated Press showing that a chief economic adviser to Mr Obama offered reassurances that anything he had said on the campaign trail "should be viewed as more about political positioning than a clear articulation of policy plans".
Even before making a campaign appearance at the University of Ohio in Toledo yesterday, Mrs Clinton gathered reporters in her hotel and urged them to give her fair treatment by investigating the memo. Substitute her name for Mr Obama's in the story, she said, and the media surely would not hesitate.
"If you come to Ohio and you go give speeches that are very critical of Nafta ... and then we find out that your chief economic adviser has gone to a foreign government and basically done the old wink-wink – 'Don't pay any attention, this is just political rhetoric' – I think that raises serious questions."
The adviser, Austan Goolsbee, has publicly denied making any such statements. However, the Clinton campaign continued to press its case, sending a stream of advisories on the issue to reporters throughout yesterday. It was also taken up by the Governor of Ohio, Ted Strickland, introducing Mrs Clinton to a crowd of about 500 supporters on the University of Ohio campus.
The memo revealed that "her opponent's not terribly serious about what he's saying in Ohio about Nafta but this is just political rhetoric", Mr Strickland said on the stage, alongside other Clinton backers, including the actor Ted Danson. "Well, we people in Ohio believe that you ought to say what you mean and mean what you say."
An aide travelling with Mrs Clinton rejected the notion that she had to win both Ohio and Texas to remain in the race. "There are no 'have-tos' tomorrow," insisted Doug Hattaway, a senior spokesman, adding that it remained a race between them for the biggest number of delegates to the party's convention.
A total of 2,025 delegates are needed for either of the Democratic runners definitively to claim victory. According to the AP count, Mr Obama currently has 1,385 delegates to Mrs Clinton's 1,276, putting her more than 100 adrift. Because of the arcane mathematics of apportioning delegates, in Texas particularly, she would have to win by wide margins today to close the gap significantly.
With Mr McCain now certain to be the nominee of his party, Democrats are growing anxious to settle their own nomination race. Without a clear set of victories tonight, Mrs Clinton may face calls therefore to withdraw in favour of Mr Obama.
"I think D-Day is Tuesday," said Governor Bill Richardson of New Mexico, who himself withdrew from the race in January. "We have to have a positive campaign after Tuesday. Whoever has the most delegates after Tuesday, a clear lead, should be the nominee."
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