Although there are nine genera of crab spiders (family Thomisidae), only three of them are known to inhabit flowers. The spider above is probably among these three: either Misumena, Misumenoides, or Mecaphesa. A clear view of the eyes would be required to positively identify the genus. Since there are only a few known species of Misumena and Misumenoides in this area, however, it’s likely that this is of the genus Mecaphesa.
Spiders in these genera sit patiently on flowers, waiting to ambush pollinating insects or other insects that happen to get too close. This spider has chosen an eastern daisy fleabane (Erigeron annuus, Asteraceae) as its hunting spot.
With their long front legs and potent venom, these spiders can bring down prey much larger than themselves. I came across some great pictures of these spiders taking down crane flies, bumble bees, butterflies, and dragonflies.