Zogby Analytics just conducted an online poll of 1,349 likely Democratic primary/caucus voters nationwide (March 3-5) and the top line results show no real surprises. In a potentially crowded field for the 2016 Democratic nomination, former Secretary of State and First Lady Hillary Rodham Clinton leads with 46%, followed by Vice President Joe Biden 13%, Massachusetts Senator Elizabeth Warren 9%, California Governor Jerry Brown 4%, New York Governor Andrew Cuomo 3%, HUD Secretary Juan Castro and Minnesota Senator Amy Klobuchar at 2%, and former Maryland Governor Martin O'Malley and former Virginia Senator Jim Webb with 1% each. One in five voters overall are undecided.

I think this is mainly good news for Mrs. Clinton. It is probably best that she is not polling over 50% because that may set a bar that ultimately is too high to achieve and the media loves to play the "expectations game" in the early rounds of the contest. She also leads among every category of Democratic voter and so she is clearly the person to beat.

But I have been doing this business a very long time and the poll does reveal some concerns for the former First Lady already. For starters, one in five Democrats (20%) is still undecided - meaning they are not convinced about a woman that has 100% name recognition. Second, her combined opposition is scoring 35%, which essentially puts the referendum on Mrs. Clinton already at 46% to 35%. One serious challenger can scarf up that 35% and only need to do a slightly better than even split among the undecided voters to make this competitive.

But thirdly, and mainly, there is possible trouble in Progressive-land for Mrs. Clinton. She now only receives 44% of self-described liberals - vs. 47% among the other candidates combined (with 9% undecided). And her numbers among First Globals (those voters born between 1979 and 1996, who number 390 in our sample) are a deficit of 47% for other candidates to her 38%, with 15% undecided.

One thing we have all learned is that Clintons can withstand the heat. And the heat is just beginning to be turned on. But the poll also reveals that one credible challenger has the potential to make a case against her nomination - something we saw already happen in 2008. And Mrs. Clinton looked a lot more fired up and ready to charge back then than she does today.