Former Gov. Mike Rounds, a Republican, has a large lead over likely Democratic challenger Rick Weiland in South Dakota’s open Senate contest, according to an automated GOP poll released on Monday.

Rounds leads Weiland, a former aide to former Sen. Tom Daschle, D-S.D., 58 percent to 38 percent, with 10 percent of respondents undecided, according to the survey from Harper Polling.

The open-seat contest marks one of the Senate GOP’s best pickup opportunities this cycle. Sen. Tim Johnson’s retirement announcement left his party searching for a top candidate to take on the former two-term governor.

Rounds is the favorite to win the GOP nomination, but he faces a primary against three other GOP candidates: physician Annette Bosworth, state Rep. Stace Nelson and state Senate Majority Whip Larry Rhoden.

Weiland fares much better against this tier of candidates, according to the poll.

Against Bosworth, Weiland leads 38 percent to 36 percent; against Nelson, Weiland trails by 2 points; and against Rhoden, Weiland trails 41 percent to 35 percent.

The poll, which surveyed 517 likely voters on Sept. 5-6 via an automated telephone survey, has a 4.3-point margin of error.

The South Dakota Senate race is rated a Lean Republican contest by Rothenberg Political Report/Roll Call.