As results began coming in on Super Tuesday, some of the most vulnerable Democratic members of Congress breathed a sigh of relief: Joe Biden was surging.

A week later, those sighs became a veritable exhalation: The former vice president won primaries in key states, including Michigan, to solidify his lead over Bernie Sanders in the all-important delegate chase.

"I'm thrilled to have Joe Biden at the top of the ticket — I think he is exactly the type of candidate we need to overcome the divisiveness of the Trump administration," Rep. Harley Rouda of California told NBC News last week. “I think he's going to help a lot of down-tickets such as myself on the ballot.”

Rouda, a first-term congressman, is part of the wave of Democratic candidates who turned traditionally red seats blue to help flip the House in the 2018 midterms. He and other "Frontliners" — what House Democrats' campaign arm is calling those members attempting to win tough re-election battles in 2020 — had been watching the party's early presidential nominating contests with keen interest and no small amount of anxiety.

For those lawmakers fighting to stay in office in districts that have elected Republicans for decades, a Democratic socialist at the top of the party's ticket felt like it would be an in-kind donation for their opponents. Many held back on endorsing any candidate. And prior to Biden's surge, Sanders' presidential bid had already emerged as a trigger point in those re-election battles, with some facing attacks of "socialism," despite their own moderate agendas.

Strategists say it’s almost impossible for House candidates to distance themselves from the presidential nominees in a presidential election year, and Sanders’ promise of “revolution” makes that all the more difficult.

“What these Frontliners are looking at is, they don’t want to spend every single day being consumed by having to answer for Sanders rhetoric or Sanders policy,” said Rodell Mollineau, a longtime Democratic strategist who is an adviser to Biden’s super PAC. “The more time — as a candidate — that you’re spending having to defend or deflect from any sort of national phenomenon is a day that you’re not talking about the issues that your constituents care about.”

Democratic Rep. Lizzie Fletcher, who represents Texas' 7th District and has yet to endorse a primary presidential candidate, hardly aligns with Sanders on politics — her West Houston district is reliant on energy jobs and she openly opposes policy positions like his fracking ban. But after Republican Wesley Hunt won his party's primary on Super Tuesday, he released an attack ad telling voters that “a vote for Lizzie is a vote for Bernie."

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Mollineau said that the distance between Sanders and Fletcher on policy hardly mattered. “If you’re explaining, you’re losing,” he said.

Strategists and candidates said Biden — who endorsed and campaigned for many of the Frontliners in 2018 as a surrogate — fits into their political brands, too.

“The way that many of these Frontliners won their elections was by not being hyper partisan, by communicating a message of working together to make the lives of constituents better,” he said.

Earlier on, as Biden struggled and Sanders soared in the first voting states of Iowa, New Hampshire and Nevada, vulnerable down-ballot candidates offered endorsements sparingly: Some threw their support behind home-state candidates or Mike Bloomberg, who was competing against Biden and others for the moderate lane in a still-crowded field of candidates.

Of the 42 at-risk Democrats in the Democratic Congressional Campaign Committee Frontline program, none have endorsed Sanders. Nine endorsed Biden before voters went to the polls on Super Tuesday, but after his wins started pouring in, the endorsements started flowing: Biden picked up 11 more nods in less than two weeks.

Rouda endorsed Biden after Super Tuesday's returns, as did Reps. Sean Casten of Illinois, Abigail Spanberger of Virginia, Andy Kim of New Jersey, Elissa Slotkin of Michigan, Haley Stevens of Michigan, and Debbie Mucarsel-Powell of Florida. After Biden's big wins in Mississippi, Missouri and Michigan on March 10, more Frontliners joined the list, with Reps. Lucy McBath of Georgia, Max Rose of New York, and Josh Gottheimer and Mikie Sherrill, both of New Jersey, adding their names to his list of endorsements on Wednesday.

Spanberger said Biden’s surge confirmed that voters still want the same thing they did when they sent her to Washington last year: “Forward movement,” not “complete upheaval.”

Rep. Colin Allred, D-Texas, one of the handful of Frontliners who endorsed Biden in January prior to any ballots being cast, said voters wanted to know “what we were actually going to do — things that were achievable.”

“I was raised by a single mother in Dallas,” he said, adding that “people like my mother don’t have time to wait for supermajorities" in Congress, something moderates have argued would be necessary for there to be any hope of passing Sanders' agenda.

The Sanders campaign argues they’re building an infrastructure that will boost down-ballot candidates by bringing new voters to the party while inspiring a political movement that will elect a new generation of Democrats, like Reps. Alexandria Ocasio-Cortez of New York and Rashida Tlaib of Michigan.

The campaign also told NBC News they’re working to build relationships with down-ballot candidates, including those who aren’t likely to endorse Sanders.

Rouda said he was never personally approached by Sanders' team, and Spanberger's and Allred's campaigns said the same.

Rep. Mark Pocan, D-Wis., who represents a reliably Democratic district and endorsed Sanders in January, argued that Democrats embraced establishment candidate Hillary Clinton in 2016 and still lost.

“Too often people forget that the best way we win elections — people like Tammy Baldwin from Wisconsin has made a career of this — is not fighting over a few people in the middle,” he said in interview, arguing that bringing new voters to the party was key to winning big.

Pressed for proof — first-time voters favored Sanders but were a small share of Super Tuesday’s voters, according to NBC exit polls — Pocan argued the nomination fight wasn’t over yet.

“Joe Biden had one good week out of the last year, and everyone wants to think that the last week is the next year,” he said. “Just looking at Super Tuesday and deciding that is the end all, be all? It’s a huge mistake.”