Agence France-Presse

HOUSTON, Texas - A bitter row erupted Friday as Democrat Barack Obama accused Hillary Clinton of scare tactics over a provocative presidential campaign ad hinting he was too inexperienced to protect US kids.

The ominous ad started airing in Texas ahead of crunch primaries there as well as in Ohio on Tuesday, which Clinton's campaign has said the New York senator must win to stay in the White House race.

In the ad, a telephone rings insistently in the background as the camera pans over small children sleeping soundly in their beds.

"It's 3:00 am and your children are safe and asleep. But there's a phone in the White House and it's ringing," the male narrator says.

"Something's happening in the world. Your vote will decide who answers that call."

Although the ad never mentions Obama by name, the implication is clear as a business-like, bespectacled Clinton answers the phone, suggesting her rival would not have the ability to protect the nation.

But Obama, on a roll after winning the last 11 nominating contests in the race to be the Democratic Party candidate in November's presidential elections, hit back, accusing Clinton of trying to scare up votes.

"We've seen these ads before. They're usually the kind that play upon people's fears and try to scare our votes," he told a campaign rally in Texas.

"The question is, what kind of judgment will you exercise when you pick up that phone? In fact, we have had a red phone moment. It was the decision to invade Iraq. Senator Clinton gave the wrong answer," he said.

Obama has been chipping away at Clinton's lead in the run-up to Tuesday's make-or-break primaries, with a Thursday poll giving the Illinois senator a 48 to 42 percent advantage over Clinton in Texas.

In Ohio, the Reuters/C-Span/Houston Chronicle poll conducted by the Zogby Institute showed Clinton ahead of Obama by just 44 to 42 percent, a lead within the poll's margin of error, making the race too close to call.

Vermont and Rhode Island are also holding votes on the same day.

Obama eclipsed Clinton yet again on Friday, burying her record fundraising with his own flood of fresh campaign cash, with The New York Times reporting he had raised 50 million dollars in February.

Clinton raised a record 35 million dollars in February, aides said, but that jaw-dropping haul was set to be dwarfed by Obama's although his camp has not released his monthly fundraising total.

Advisors to the former first lady said however that the torrent of money flooding into campaign coffers left her well positioned to take on her Democratic adversary in Ohio and Texas.

"Hillary's going to one place, to Denver as the nominee of the Democratic Party to be president of the United States of America," her campaign chairman Terry McAuliffe said, referring to the party's nominating convention in Colorado.

And even though some sections of the media seem to have already anointed Obama as the Democratic presidential candidate, the senator himself denied any over-confidence heading into Texas and Ohio.

"Remember New Hampshire," he told reporters aboard his plane, recalling Clinton's come-from-behind victory that revived her candidacy last month.

On the Republican side, John McCain for one is already anticipating a White House match-up with Obama, as he poised to eliminate former Arkansas governor Mike Huckabee and clinch the Republican nomination on Tuesday.

Yet even Obama's campaign director, David Plouffe, suggested he believed the drawn-out tussle with Clinton would carry on beyond Tuesday.

"The most likely outcome is not a huge delegate swing," he told reporters, referring to the battle to win over the delegates who will crown the party's nominee at its August convention.

On Friday, Obama remained slightly ahead of Clinton in the delegate count with 1,384 delegates to her 1,279.

"The Clinton campaign believe they need to win both Texas and Ohio by over 10 points ... we're running close contests in both of those states," Plouffe said. "They're going to fail miserably by that measure."