South Dakota Democrats are breathing a sigh of relief this weekend after likely avoiding a contentious primary fight to succeed retiring Sen. Tim JohnsonTimothy (Tim) Peter JohnsonSeveral hurt when truck runs into minimum wage protesters in Michigan Senate GOP rejects Trump’s call to go big on gun legislation Court ruling could be game changer for Dems in Nevada MORE (D-S.D.).

Rick Weiland’s decision to enter the Senate race is seen as a sign that U.S. Attorney Brendan Johnson — the retiring senator’s son — will not enter the race.

Weiland and Johnson are both progressives, and political observers in the state said Weiland almost certainly would not have entered the race if Johnson were still considering a run.

Ex-Rep. Stephanie Herseth Sandlin (D) is widely expected to enter the race and will make a decision by the end of the month, sources said.

The Democratic Senatorial Campaign Committee (DSCC) had polling showing Herseth Sandlin, with her more conservative voting record, would be more competitive in a general election against former Gov. Mike Rounds (R), the only announced Republican contender in the race.

Weiland would be a heavy underdog against Herseth Sandlin, and could ease her path to the general election, where she would be likely to face Rounds in the deeply red state.

“A primary between her and Weiland would not be problematic and challenging in the way that a primary between her and Brendan would be,” one national Democratic operative said.

Weiland lost to Herseth Sandlin in a four-way primary during his 2002 run for Congress by more than 25 percentage points.

Weiland said he’d characterize himself in another race against her as the true defender of Democratic Party values. He’d likely frame Herseth Sandlin, in contrast, as too conservative, and highlight her votes against ObamaCare, her opposition to gay marriage and her pro-gun stance.

“There was a real concern, they wanted to ensure that we had a candidate in the race that stood up for our Democratic principles,” he said, when asked for his rationale for running.

Weiland has also hired Mike Lux, a progressive strategist with ties to left-leaning groups like Americans United for Change and Progressive Majority, as an adviser and will traveling to Washington in the coming weeks to meet with, he said, potential supporters.

Herseth Sandlin’s votes on hot-button issues make national Democrats believe she’d be the best candidate to keep the seat, running in a state where President Obama lost by nearly 20 percentage points in 2012.

A primary that frames a candidate as a centrist in their party can be beneficial to that candidate’s chances in the general. In the Massachusetts special Senate election, Republican nominee Gabriel Gomez faced a challenge from Tea-Party-backed Michael Sullivan in the primary, and Gomez was hammered early on for proposing that he’d support President Obama’s agenda.

Republicans believe, however, that that line of attack will actually help Gomez in the general, running in a state as blue as Massachusetts.

And Weiland’s entrance into the South Dakota race makes it more likely that Republicans will face a potentially bruising primary as well. Conservative Republicans are frustrated with Rounds’s record and have been looking for a challenger, though none has yet emerged.

But a source familiar with Rep. Kristi Noem’s (R) thinking said the news that Democrats may face a contested primary “will certainly pique” the Tea Party favorite’s interest in the race.

“This could potentially make our path to a run for Senate easier,” the source said.

The source added that Rounds’s paltry first-quarter fundraising haul — she raised about $184,000 — had also increased Noem’s interest in the race, and she’s expected to have a decision made on her potential run soon after Labor Day.

Fierce fights between the Tea Party and establishment wings of the GOP were thought to have cost the party winnable seats in Missouri and Indiana in 2012.

Weiland will, however, force Herseth Sandlin to spend money and effort she could’ve otherwise reserved for a general election fight, and there remains a wildcard in the primary in former Senate Majority Leader Tom Daschle (D).

Weiland told The Hill he expects Daschle to campaign with him. Daschle gave a glowing statement on Weiland to the Argus Leader, saying he encouraged him to run “with great enthusiasm.” Weiland said the two had a “heart-to-heart” in which Daschle was “very excited I was even thinking about getting back into public service.”

Daschle’s campaigning for Weiland could improve his chances in a primary, as he’s well-liked and well-known throughout the state, and could prove a powerful fundraising force and surrogate on the trail.

However, former Daschle aide Steve Dick said he wasn’t sure if Daschle’s help would “put [Weiland] over the top” in a primary. He also noted that it’s unclear exactly how the race will unfold, as South Dakota hasn’t seen a similar situation for decades.

“We haven’t had an open Senate seat since 1978. We've either had incumbents get reelected or incumbents get defeated. It remains to be seen yet exactly how this is going to play out,” he said.

National Republican Senatorial Committee communications director Brad Dayspring noted the complications that could arise for Democrats if the primary pits Daschle against the DSCC.

"Rick Weiland is a true progressive who believes completely in the Reid/Schumer Agenda. This ugly primary race pits Tom Daschle against the DSCC, making Weiland and his fundraising ability a major headache for Washington Democrats who were sure they hand pick Herseth Sandlin," he said.