The indictments will affect his popularity and performance for sure, but he's practically a religion in Laredo and surrounding areas. Unless he resigns or gets expelled, he'll win and outrun Biden.
As a Texan I'm assuming you're somewhat more familiar with the local situation...is Cuellar really
that crazily popular in the Laredo area? I mean, yes, the numbers don't lie and he did massively outperform Biden in those counties, but keep in mind that it was still on par or behind Obama's 2012 performance in those areas, so it's not necessarily just about his personal popularity.