NBP/NPN, July 23-28, 2014:
Rounds: 43%
Weiland: 30%
Pressler: 14%
Howie: 4%
This appears to be an independent poll.
While Rounds has a continuous, clear lead in this senate race, he should be concerned about one thing: The last poll to show him with 50% or more support, aside from that crappy yougov poll, was all the way back in February. If Pressler's/Howie's support drops and goes disproportionately to Weiland, this race will become (a lot) closer than it should be, unless Rounds can suck some current Weiland voters out of the Weiland column.
The 50% rule isn't really a thing. Margin matters a lot more, and since SD doesn't have a runoff for the GE, Rounds will win this. It may be closer than expected but follow the money and you'll have your winner. Neither the DSCC nor the RSCC are spending here because Rounds will win by double digits and Weiland has no path to victory.