Hillary Clinton is nothing if not committed to winning this presidential contest. As she sees the clock wind down, though, Clinton has to pull out all the stops to pull out what seems like an increasingly improbable victory. Now, with the aid of veteran ad man Roy Spence, she has done so, as ABC's Nitya Venkataraman reports.

In a new ad (complete with Stephanopoulos analysis), set to air in the closing days before crucial Texas and Ohio primaries, Clinton takes half a page from Lyndon Johnson and half a page from Walter Mondale to suggest, again, that Barack Obama is not ready to be president.

"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 happened in the world," a deep-voiced narrator intones, as children sleep in the dark. "Your vote will decide who answers the 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. It's 3 a.m. and your children are safe and asleep. Who do you want answering the phone?" Clinton, answering a phone, says she approves the ad.

The spot is perhaps the toughest Clinton can be without overtly claiming an imminent terrorist attack. Reminiscent of Johnson's "Daisy" ad in 1964 and Mondale's 1984 primary ad, depicting a shrieking red telephone and asking voters who they wanted on the other end.

It's no wonder comparisons should be made to Mondale's spot, which Gary Hart later credited for seriously damaging his credibility. The two spots are very similar, and Spence, who came on board the Clinton team after New Hampshire, created both.

Some will claim the ad is over the top, but Clinton had to emphasize her experience somehow. In a way, it's surprising that it's taken this long; that with just a weekend left to get her message out, she's waited until now to make her case in the strongest way possible.

The ad is certainly no "Daisy." "These are the stakes: To make a world in which all of God's children can live, or to go into the dark. We must either love each other, or we must die," Johnson thunders over a nuclear fireball. "Daisy" was so controversial that it ran a total of one time. Even Mondale's ad, which lasted longer than a single run, had a sharper tone.

Political admaking, in short, is becoming more subtle, and at the first hint of overtness, many cry wolf. That limits a candidate's ability to make up big deficiencies quickly. Still, if Obama prevails in the primary, expect a much harsher version of the ad, with John McCain offering the tag line.

Update: Obama chief David Plouffe hit back hard in a morning conference call with reporters. "Senator Clinton had her red phone moment. She had it in 2002. It was on the Iraq war. And she and John McCain and George Bush all gave the wrong answer," Plouffe said. "This is about what you say when you answer that phone."