With polls painting Texas as the big wild card in Tuesday's critical Democratic primaries, Hillary Clinton is wielding the national security hammer to beat back Barack Obama in the Lone Star State and keep her campaign alive.

Clinton pulled the fear card Friday, launching a new ad playing only in Texas that shows images of sleeping children and asks voters who they want "answering the phone" during a crisis.

She also held a somber rally in Waco, Texas, where, flanked by retired generals and other veterans, she stressed the importance of the U.S. military and the need for a responsible commander in chief.

The emphasis on national security comes as surveys show Texas voters are more attuned to the issue than voters in Ohio. Both states vote March 4, and both are considered must-win for Clinton.

But a new FOX News poll shows Texas -- and its goldmine payoff of 193 delegates -- is a dead heat, making it the likely battleground as Clinton maintains a modest but consistent lead in Ohio. The polls, taken from Feb. 26-28, showed Clinton with a 46-to-38 percent lead in Ohio, and Obama with a slim 48-to-45 percent lead in Texas. The margin of error was 4 points.

With her military push, Clinton is trying to exploit her opponent's alleged weakness on experience and change the dynamic in the final days before the Texas vote.

"We need a president who is tested and ready to lead in a dangerous world and who can draw on years of real-world experience," Clinton said at the Waco rally. "There's a big difference between giving a speech on national security and giving orders as commander in chief."

The FOX News poll showed Texas voters, unlike Ohio voters, place the Iraq war high among their list of top issues, just below the economy.

Clinton's new ad Friday took the rare step of using the threat of terrorism against a fellow Democrat, and was immediately dismissed as fear mongering by the Obama camp, which answered back with a nearly identical ad later in the day.

The Clinton ad begins with a somber voiceover that says: "It's 3 a.m. and your children are safe and asleep. But there's a phone in the White House and it's ringing. Something's happening in the world.

"Your vote will decide who answers that call ... whether it's someone who already knows the world's leaders, knows the military -- someone tested and ready to lead in a dangerous world."

After panning across shots of babies and children sleeping in a quiet suburban setting, the ad cuts to Clinton answering that ringing phone.

Obama, who has touted his long-standing opposition to the Iraq war as evidence of his foreign affairs judgment, accused Clinton of trying to "scare up" votes with the ad and said she had her "red phone moment" and failed when she sided with President Bush on the Iraq war.

Obama plans to air his response ad starting Saturday in Texas. The Obama ad practically lifts the script from Clinton's spot, but says "When that call gets answered, shouldn't the president be the one -- the only one -- who had judgment and courage to oppose the Iraq war from the start."

Observers said the Clinton ad raised a fair and potentially potent point.

"I think it's a terrific ad. I think it goes negative against Obama without attacking him directly," said pollster Doug Schoen, adding that it effectively brings into question who is best equipped to handle a crisis.

Dan Palazzolo, political science professor at the University of Richmond, said the ad could be the campaign's "last card," but that for it to be effective she's got to hammer the same themes on the stump.

Her campaign is doing just that. Retired Gen. Wesley Clark introduced Clinton in Waco by stressing the new ad and urging voters to watch it.

Clinton followed up, saying: "I also understand completely what it means when that phone rings at 3 a.m. There isn't any time to convene your advisers, to do a survey about what will or will not be popular. You have to make a decision."

She accused Obama of being "missing in action," by not following up with action in the U.S. Senate on his 2002 speech opposing the Iraq war.

Texas is still a toss-up.

The two-step primary-then-caucus voting process in the state could favor Obama, since the Illinois senator has proved adept at successfully navigating the caucus system. But the FOX News poll showed voters are fluid -- one in five Obama voters said they may change their mind. Slightly fewer said so for Clinton.

Should Texas become Clinton's Alamo, even her aides say she will have to make a decision on whether to push forward. Associated Press delegate tallies show she's trailing Obama by about 100 delegates.

Obama campaign manager David Plouffe predicted on a conference call Friday that Clinton will ‘"fail" at making a big dent in the campaign's pledged delegate lead, which he estimated at more than 160.

"They have one task for March 4, and that is to seriously erode our pledged delegate lead," he said. "They are going to fail and fail miserably."

Clinton's campaign hit back with a memo claiming Obama must win all four states voting March 4 -- Ohio, Texas, Rhode Island and Vermont -- because he's outspent her in paid media and put more resources and staff into the states.

"If he cannot win all of these states with all this effort, there's a problem," the memo said.

West Virginia Sen. Jay Rockefeller, chairman of the Senate Intelligence Committee, endorsed Obama Friday, citing his position on the Iraq war as a sign of his judgment.

Besides the two-step voting process, early voting is another variable that could significantly turn the results.

Both campaigns have urged supporters to participate in early voting, which ends Friday in Texas.

Clinton reminded supporters to do so in Waco Friday afternoon. Texas Democrats had already shattered early voting turnout records earlier in the week.

As of Wednesday, 584,994 Texans in the state's 15 most populous counties had voted Democratic.

Clinton's campaign also tried to get traction Friday out of a report that aired on the Canadian television network CTV citing anonymous sources saying a senior Obama adviser called Canadian ambassador Michael Wilson to warn him that Obama would criticize NAFTA , but that it was just campaign rhetoric he doesn't believe.

The embassy and Obama campaign denied it, but the Clinton campaign complained Friday that Obama hadn't answered questions from a follow-up report about an alleged conversation on the same matter between a senior adviser and the Canadian consulate in Chicago.

The original CTV report also implicated Clinton, saying her campaign was "indirectly" contacting the Canadian government to stress her commitment to NAFTA no matter what she says.

FOX News' Major Garrett, Aaron Bruns, Bonney Kapp and Judson Berger and The Associated Press contributed to this report.