The cornerstone of Hillary Clinton's strategy over the course of the campaign has been to contrast her greatest perceived strength - experience - with the relatively thin resume of Senator Obama. Yet despite being on her fourth or fifth iteration of this message, she's clearly failed to make the case with Democratic primary voters that her experience makes her a significantly better choice than Obama.

So why has she failed? It's not because Democrats don't value experience. Indeed most polls suggest Democrats consider experience nearly as important as their desire for change, and Clinton has maintained large advantages over Obama on the question of experience throughout the primary season.

What Clinton has failed to do is to frame the experience question in a way that resonated with Democratic voters. "Ready on day one" is a catchy enough phrase, but it puts the emphasis in the wrong place by making the contrast with Obama implicit rather than explicit.

Put another way, Hillary never needed to convince Democratic voters that she was "ready to lead" - she's the most well known Democratic presidential candidate ever. She needed to convince voters that Barack Obama was not.

To do this effectively Clinton should have been reminding voters over and over that just four short years ago Barack Obama was in the Illinois state legislature (voting present, no less) and now he's running to be the leader of the free world. Put in that context, the magnitude of the jump Obama is seeking to make suddenly gains perspective, and Clinton has voters pondering exactly the question she wants: wow, is he really ready for this?

Clinton has also done a miserable job of leveraging her experience as First Lady against Obama, even allowing him to question its legitimacy on occasion. Again, it's not enough for Clinton to tell voters about her accomplishments as First Lady, but to frame them in way that makes Obama look embarrassingly small by comparison. Something like:

"Thirteen years ago, before Barack ever though about entering public life, I was challenging the Communist dictatorship in China over the issue of human rights and helping broker peace in Northern Ireland. While he was busy voting present in the Illinois state legislature, I was working to tackle serious issues across America and around the globe."

Translation: Clinton's been playing on the world stage for years. Obama's been playing in Springfield, Illinois.

The beauty of this strategy is that it's not what most people would characterize as "going negative." Clinton would certainly be demeaning Obama's experience publicly, but that's the point: to keep Democratic voters focused on the thinness of his accomplishments prior to entering the Senate.

And if Senator Obama and his campaign bristled at the framing or the tone of the comparisons, so much the better. Clinton would still have shifted the discussion to make Obama battle on her terms and her turf.

Instead, Clinton has allowed Obama to waltz through this campaign touting his experience in the Illinois state legislature as if it were on par with being in the United States Senate or even with Clinton's experience as First Lady. Without an aggressive effort by Clinton to remind Democratic voters of the quantum (or perhaps we should say "audacious") leap Obama is seeking to take, Obama has sufficiently blurred the distinctions on the question of experience and at the same time used his inspiring rhetoric to focus voters' attention elsewhere.

The problem for Clinton is she should have started framing the experience question months ago. With only a few days left until a crucial vote in Ohio and Texas that could effectively eliminate her from the race, it's probably too late.