The movement appears subtle, but Gallup notices a shift away from Barack Obama over the past week. For the first time in the cycle, the split between John McCain and either Democrat has shown a different result — and now Hillary Clinton looks slightly stronger than her opponent in the general election. That may provide her with more ammunition for her argument to superdelegates to support her despite the popular vote and pledged delegate split:

The latest Gallup Poll Daily tracking update on registered voters’ general election preferences for president finds that John McCain has opened up a slight advantage over Barack Obama, 47% to 44%, while McCain and Hillary Clinton remained tied at 46% each.

Those results are based on interviews with over 4,000 registered voters nationwide conducted March 11-15. This marks the first time since Gallup began reporting these general election results last week that McCain’s relative positioning against Obama and Clinton has not been exactly the same. McCain’s three percentage point advantage over Obama is not statistically significant, and it remains to be seen if the differentiation between the two Democratic candidates, when pitted against McCain, will continue in the days ahead.

The results didn’t exactly look good for Hillary, either. She simply didn’t lose as much ground to McCain as Obama did. Both Democrats started the week with a +2 against the Republican nominee (46-44 Obama, 47-45 Hillary). Hillary ended it in a 46-46 tie, while Obama’s five-point turnaround left him at the short end of a three-point gap.

If Hillary wants to make the electability argument, though, she may find a less-than-receptive audienc. The same Gallup poll that puts Hillary in a relatively better position against McCain has her at a disadvantage to Obama. It has improved since being six points down mid-week, but she trails Obama among Democrats by three points.

Why do Democrats rate her lower than Obama while voters overall give her a slightly better head-to-head performance against McCain? It could be that fewer Democrats will defect if Hillary gets the nomination than will if Obama wins it. That may be another argument for Hillary with the superdelegates in Denver. Which candidate can hold more Democrats in the fold, even if not winning the majority of primary voters overall?