"I am going to do everything I can to help Rick.  I hope you will too," he says in a fundraising email sent to supporters by the campaign on Monday.

Daschle writes that he spoke with Weiland about Herseth-Sandlin's decision not to run, and that Weiland "told me, and he is right, that her decision puts an even heavier burden on him to be a strong, effective voice for our state, and our values, in the Senate election next year."

Daschle had previously encouraged Weiland to run, and expressed enthusiasm for his candidacy, but until the email, he hadn't outright endorsed the candidate.

Weiland remains the underdog in the race, but he's currently Democrats' only candidate for retiring Sen. Tim JohnsonTimothy (Tim) Peter JohnsonCourt ruling could be game changer for Dems in Nevada Bank lobbyists counting down to Shelby’s exit Former GOP senator endorses Clinton after Orlando shooting MORE's (D) seat. Former Gov. Mike Rounds (R) is the only announced Republican candidate.

Daschle, who is still well-liked and well-known in South Dakota, could help boost Weiland by campaigning and fundraising for him.

Weiland does not yet have the backing of national Democrats, however — the Democratic Senatorial Campaign Committee hasn't endorsed him, and declined to comment when he announced his candidacy, an indication he's not their first choice.

DSCC spokesman Justin Barasky issued a statement following Herseth Sandlin's announcement that pointedly avoided mentioning Weiland.

"There will be a strong democratic candidate that can seize on the divisive GOP primary and provide South Dakotans with a clear alternative to the dysfunction on the Republican side," he said.