Barack Obama bids on Tuesday to knock Hillary Clinton out of the White House race after a mud-slinging campaign that Democratic grandees fear is helping nobody but Republican heir John McCain.
But heading into crunch battles in Ohio and Texas, the former first lady is full of fire and has been eviscerating her charismatic rival's qualifications to be commander-in-chief and chief steward of a troubled economy.
While the Democrats fight it out, McCain looks set to lock up the Republican nomination by eliminating former Arkansas governor Mike Huckabee.
Aside from Ohio and Texas, Rhode Island and Vermont also hold presidential primaries on Tuesday.
The day's voting will kick off in Vermont at 1000 GMT and will end in Rhode Island at 0200 GMT on Wednesday -- although a quirky electoral process in Texas will see caucuses start after the Lone Star state's day-long primary.
Clinton said Tuesday she was just getting "warmed up" after the longest and costliest primary race in history, and her campaign has bared its teeth in attacking Obama's credentials on trade and national security.
But since the original Super Tuesday of February 5, Obama has been on a roll with 11 nominating victories in a row, and now enjoys a small but clear lead in the Democratic delegate count.
"If we do well in Texas and Ohio, I think the math is such where it's going to be hard for her to win the nomination, and they'll have to make a decision about how much longer they want to pursue it," he told ABC News.
Pressing home his financial advantage after a banner month of fundraising in February, the Illinois senator aired a two-minute ad in Texas that detailed his extraordinary life story and highlighted his opposition to the Iraq war.
Obama leads in Texas by 46-45 per cent in his quest to be the first black president, according to polling by McClatchy Newspapers, MSNBC television and the Fort Worth Star-Telegram.
A Quinnipiac University survey Monday had Clinton up 49 to 45 per cent in Ohio, but her lead had shrunk dramatically from a 21 point gap on February 14.
New Mexico Governor Bill Richardson, a top Democrat who has stayed neutral in the race, argued Sunday that whoever emerges on top after Tuesday should be anointed the party's standard-bearer.
His call is likely to turn into a tidal wave if Clinton loses Ohio and Texas.
According to RealClearPolitics.com, Obama leads by 1,392 delegates to Clinton's 1,279, and Tuesday's voting is unlikely to do much to change that arithmetic.
McCain is favored to vanquish Huckabee's challenge on Tuesday because the Republicans award all their delegates to a state's winner.
But the Democratic voting is proportional, meaning that Clinton would need landslide victories on Tuesday and beyond just to pull even with Obama.
Senator John Kerry, the party's defeated 2004 candidate backing Obama, said Clinton must decide after Tuesday whether she has any path to the nomination.
"If she doesn't win Texas, the former president (Bill Clinton) himself has said that he thinks it would be very difficult to continue," he said.
"But I do think that if you're going to measure it by their own standards, it's pretty important not just to win, but to win big. Sneaking by does not change the fundamental dynamics of this race."
Kerry also lashed out as Clinton aired ominous television spots in Texas attacking Obama's capacity to safeguard the United States from attack -- evidence, he said, that her campaign was now "grasping at straws."
But Clinton aides said it was high time for Obama to face tough questioning over who is best prepared to take on the Republicans in November and then lead the United States into an uncertain economic future.
"If he can't compete with us on who can be commander-in-chief, who can be a steward of this economy, he can't compete with John McCain on these issues," Clinton spokesman Howard Wolfson said.
As the national security debate played out in Texas, the two campaigns waged running battles over the North American Free Trade Agreement in economically ravaged Ohio, where NAFTA has become a four-letter word.
Clinton accused Obama of savaging the trade deal linking the United States, Canada and Mexico in public, but going behind the backs of Ohio voters to assure Canada his warnings were mere political rhetoric.
Amid angry denials from the Obama camp, Canadian Prime Minister Stephen Harper was forced to say he regretted aspersions cast on the Illinois senator by an internal government memo.