Biden so far has support from Democratic challengers Mark Kelly in Arizona, Sara Gideon in Maine and Amy McGrath in Kentucky. But insurgent liberals, Betsy Sweet in Maine and state Rep. Charles Booker in Kentucky, endorsed Sanders, setting up ideological primary challenges against party-endorsed candidates.

Others are holding their fire. Sen. Gary Peters (D-Mich.), whose state holds a critical primary next week, didn’t weigh in on a question about Biden versus Sanders.

“I will run with either one,” Peters said. “I run on my record and run on the issues that I care about that are important to Michigan.”

But most party operatives agree that Biden is better for the fortunes of downballot candidates.

“Biden puts us in a strong position in the suburbs, which is how we won back the House in 2018 and how we're going to win back the presidency in 2020,” said Martha McKenna, a veteran operative who ran the Democratic Senatorial Campaign Committee independent expenditures in 2018. “Senate candidates should benefit from that.”

Republicans aren’t expressing much concern about the Senate landscape after riding a wave of Sanders-related attacks in recent weeks. They argue Biden is also a flawed nominee, and that even though they’re on defense, the map tilts heavily into Republican territory.

“When you look at all the previous Democratic candidates they were just different flavors of far-leftism,” said National Republican Senatorial Committee Chair Todd Young (Ind.) “Now we have Joe Biden, who proposes a $4 trillion menu of goodies for the American people, which, of course, are going to have to be paid for through middle-class tax increases.”

Republicans also got a bit of long anticipated good news this week: the NRSC took an “anybody but Roy Moore” stance in the Alabama Senate primary, and Moore, the controversial judge who lost a 2017 special election, finished a distant fourth Tuesday. Former Attorney General Jeff Sessions and former Auburn University football coach Tommy Tuberville advanced to a March 31 runoff, and party leaders feel confident about either candidate's odds to oust Democratic Sen. Doug Jones in November.

Some Democrats urged caution about reading too much into positive developments. Bullock is not officially in the race yet, though he is widely expected to run. Biden hasn’t secured the nomination, and their preferred candidates still face primary challenges.

Sen. Brian Schatz (D-Hawaii) said his advice was to treat it like golf: “Don’t get too excited when you hit a bad shot. Don’t get too excited when you hit a good shot.”

In Colorado, former Gov. John Hickenlooper, the DSCC-endorsed candidate, did not endorse before the state's presidential primary this week. He faces a Senate primary in June against a number of candidates, including former state House Speaker Andrew Romanoff. Sanders won the Colorado primary Tuesday.

Dan Baer, a State Department alum who was running for Senate in Colorado but dropped out to endorse Hickenlooper in the race to face vulnerable GOP Sen. Cory Gardner, argues Biden is clearly a better candidate down ballot, but not for ideological reasons.

“A Biden campaign would be a referendum on Trump,” Baer said. “Bernie’s campaign would be a referendum on a very different vision for our society. My gut, looking back at 2018, is that the former is better for Senate candidates in swing states.”

Democrats were also buoyed by their first primary results for their DSCC-endorsed candidates. Cal Cunningham won comfortably in North Carolina, a top target for Democrats, and MJ Hegar advanced to a runoff in Texas.

But the North Carolina effort was expensive, with more than $11 million in outside spending to boost Cunningham alongside his campaign's efforts. Sen. Thom Tillis emerged with a massive cash advantage.

“They were forced to spend a fortune to push Cunningham over a field of nobodies, and now they’ll have to spend three times that amount to try to drag him through the general election,” said Steven Law, who runs Senate Leadership Fund, a super PAC that financed $3 million in ads boosting Cunningham’s Democratic opponent in the primary.

In Texas, Hegar's May 26 primary runoff with state Sen. Royce West will give GOP Sen. John Cornyn — who already has more than $12 million cash on hand — two more months to increase his financial advantage while Democrats spend resources in the massively expensive state.

And in Montana, Republicans have long been planning for Bullock to enter the race against first-term GOP Sen. Steve Daines. Senate Leadership Fund, a super PAC aligned with Majority Leader Mitch McConnell ran ads hitting Bullock last May in anticipation of a Senate run. The state GOP released a video compilation this week of his statements dismissing a campaign for Senate.

“Gov. Bullock isn’t actually interested, nor does he have the skills or abilities to be a U.S. senator. He said so himself. He simply couldn’t resist the pressure from Barack Obama and Chuck Schumer,” Chairman Don Kaltschmidt said.

But Bullock potentially jumping in the race is a big win for Democrats, who argue he can turn it into a toss-up. Former Sen. Max Baucus (D-Mont.), who held the seat before resigning in 2014 to become ambassador to China, said that Bullock's meeting with Schumer played a key role in his reconsidering.

“Probably Steve realizes we’re in striking distance of winning back [the Senate], and, second, it’s patriotism. It’s the right thing to do,” said Baucus, who has encouraged Bullock to run and endorsed Biden's presidential bid. “It’s more difficult if Sanders is on the ballot. But with the vice president [as] our nominee [it] makes it easier.”

Still, he said, beating Daines won’t be a “cake walk. It’s going to be a very difficult race.”