To determine that, let’s look at the recent history of non-Presidential top-of-the-ballot races in Texas:
(I’m throwing out the 4-way 2006 gubernatorial race as not germane. All numbers via Wikipdia, % rounded.)
In terms of financial resources, Sadler’s fundraising has been very poor, having raised just over $800,000; poorer even than Rick Noriega, who managed to raise over $4 million, or Barbara Ann Radnofsky, who raised about $1.5 million. Then again, money isn’t everything: Tony Sanchez spent $60 million of his own money to garner a meager 40% against Rick Perry in 2002.
I don’t think Sadler will do as poorly as vanity candidate Gene Kelly did in 2000. I expect Obama to run 4-5 points worse than he did in 2008, and I expect Sadler to probably lose a point or two off Obama’s total. So, I predict: Ted Cruz gets 61% of the vote, and Paul Sadler gets around 39%.