Former South Dakota Gov. Mike Rounds raised more than $600,000 for his Senate campaign in the second quarter of this year, confirmed a spokesman for the Republican’s campaign.

Sen. Tim Johnson, D-S.D., is retiring, and his seat is a top pickup opportunity for Republicans. The national GOP has touted Rounds as its strongest candidate in the race. 

But Rounds raised just $184,000 in the first quarter of this year, making his new fundraising report one of the most anticipated Senate race filings. On Tuesday, Rounds revealed his second-quarter haul — a strong showing that lifted his fundraising total to more than $1 million since he announced his candidacy in November.

Overall, Rounds has had a good couple of months. Two top prospective Democratic candidates declined to run for the Senate seat. Former Senate aide Rick Weiland is the only Democrat in the race and has yet to report his second-quarter fundraising.

Additionally, Rep. Kristi Noem, R-S.D., decided not to run for Senate. She would have been a formidable primary challenger for Rounds.

The second quarter spans from April 1 through June 30. Campaigns must file their reports with the Federal Election Commission on or before the July 15 deadline.

Rothenberg Political Report/Roll Call rates this race as Lean Republican.