The Rothenberg Political Report sheds some light on this:
Of course Warner will endorse and back Webb in the race, even though the Senate nominee opted to withhold a potential 2008 endorsement for his fellow Virginian on NBC's Hardball. But if Warner invests significant time and energy into the race and helps Webb defeat one of the Republicans' top 2008 presidential contenders, it could vault the former governor ahead of his rivals going into 2007. But if Webb is viewed as "Warner's candidate" and loses to Allen, he would remain with the rest of the pack
Update from the DSCC
VIRGINIA: Jim Webb, coming off Tuesday+óGé¼Gäós primary win, narrowed George Allen+óGé¼Gäós lead to just 10 points, 51% - 41% in a new Rasmussen poll. Allen led Webb by 20-points, (50-30) in late April