(CNN) A little more than 15 months ago, Democratic businessman Harley Rouda would have seemed like an odd choice to help lead the so-called resistance against President Donald Trump.

But a lot has changed since Trump's victory -- starting with Rouda's party affiliation. A Republican until 1997, Rouda spent the next two decades as an independent. He and his wife donated to longtime family friend, Ohio Gov. John Kasich, during the Republican nominating contest. Rouda registered as a Democrat shortly after the 2016 election (he voted for Hillary Clinton, former President Barack Obama twice and Democratic nominee John Kerry before that, he said). Then, in March 2017, Rouda entered the congressional primary in his adopted home, California's 48th Congressional District, where Republican Rep. Dana Rohrabacher is the incumbent.

He grabbed the attention of local activists with his early support of "Medicare for All," which helped him win the twin endorsements of the California Nurses Association and National Nurses United, organizations best known politically for their close alliance with Vermont Sen. Bernie Sanders.

On Wednesday, Rouda will get another boost when Indivisible -- the influential anti-Trump resistance group -- endorses him, a booming bat signal to Democrats ahead of June's "jungle primary," which advances the top two vote-getters regardless of party affiliation.

The support of Indivisible, which in less than two years has become one of the country's leading grassroots progressive organizations, confirms the Ohio-born Rouda as the Democratic favorite in the district, which like so many others in the deep blue state was initially inundated with enthusiastic, tenderfoot candidates -- enough that Democrats in other races still risk splitting their primary support, effectively canceling each other out, and inadvertently clearing the way for two Republicans onto the general election ballot.

With Indivisible's backing, Rouda moves to the forefront of a new wave of Democratic activism which has largely sought to cultivate a more ideologically progressive — and geographically diverse -- slate of candidates heading into the midterm season.

"They do a better job than anyone else out there helping to provide talking points in how to properly position progressive issues. I think Our Revolution does a good job, as well," Rouda said, also making note of the Sanders-inspired political organization, which has not endorsed in the race. "You know, the mainstream Democratic Party has not been as aggressive in that language and those talking points, but that's from a sense of caution and wanting to make sure they're not pushing certain voter segments away."

"At that time (I entered), the Democratic Party was telling candidates not to be that forthright. Don't say 'Medicare for all,' say 'universal health care," Rouda recalled.

María Urbina, Indivisible's national political director, is the more seasoned operative charged with the delicate work of helping direct and advise the project's independently run local outposts. California's 48th district will, after Wednesday's round of endorsements are rolled out, be home to one of 15 contests with nationally backed Indivisible candidates. Those hopefuls won't receive a direct cash infusion, but the group's stamp of approval -- as voted on by its members -- is expected to key up individual donations and activist energy around the campaigns.

"One of the things we ask after you vote (to endorse nationally) is would you be willing to volunteer, and we build out that volunteer list, which is really one of the key features of the program -- that we can help with volunteer recruitment," Urbina said. "Ideally we're helping them to build capacity through this program."

The process of testing the candidates, challenging them to develop and better articulate their agendas, she added, has also helped cultivate thousands of more informed, effective supporters. The constituent town halls and policy explainer sessions, she said, "have created these super, hyper electoral influencers" -- like Indivisible Orange County 48 chairman Aaron McCall, who arranged four debates ahead of the local group's endorsement of Rouda, sorting through the candidates with something called the Voter Support Score, which ranks contestants based on a combination of total cash on-hand, money raised inside the district and total number of in-district donors.

The local-national model embraced and built up by Indivisible has become increasingly popular among Democrats stung by the 2016 presidential result and a decade of down-ballot losses that many believe helped seed Trump's stunner. Endorsements from new groups, like Our Revolution and Justice Democrats , founded after the election, along with familiar or growing outfits like the national Working Families Party and Democracy for America , are increasingly in-demand -- both as a signal to voters unaccustomed with engaging down the ballot and those looking for expert advice and elementary aid in grassroots organizing.

"The Working Families Party endorsement is like a good housekeeping seal of approval for progressive voters," said Joe Dinkin of the WFP. The party, which began as a local endeavor in New York in 1998, has grown steadily since but surged in 2017 by organizing, by its own estimate, something like 1,000 "Resist Trump Tuesdays" -- often a joint effort with MoveOn.org and other groups -- in the first 100 days of the administration. It also provides most candidates who win its local endorsements some combination of campaign consultation, training and voter contact strategies.

But a hotly energized base and bumper crop of new candidates comes with complications.

The party's House campaign arm, the Democratic Congressional Campaign Committee, has come under intense scrutiny from progressive activists and media for its reported tactics in thinning unusually crowded primary fields. The grumbling is, in part, a question of ideology -- with the left arguing the party should back more progressive candidates -- and older frustrations, a feeling that the DCCC has ignored races it viewed (and dismissed) as lost causes while effectively ceding large swaths of the country to Republicans.

DCCC national press secretary Tyler Law argued that Democrats don't have to choose between the party and the new grassroots groups.

"Voters across the country have been working hard every day to hold House Republicans accountable and flip districts blue, and the DCCC has long recognized and appreciated the unprecedented influence that the grassroots have in these races," he said. "As we've indicated all cycle, the DCCC is working closely with our allies, including the many new grassroots organizations, to ensure that there's a competitive Democrat on the ballot for voters to elect in November."

Despite its efforts to ramp up grassroots activity , an effort backed by truckloads of cash raised online from more than 340,000 first-time donors, according to Law, the party brand remains in the tank , complicating an already touchy process. Even within some districts that the DCCC and groups like Indivisible or Our Revolution agree are prime to be flipped, the party and the grassroots organizations have endorsed different candidates.

Pennsylvania Democrat Jess King, who is running unopposed now due to the state's contentious redistricting process in its 11th Congressional District, remain skeptical of the party program. Her conversations with the DCCC and Our Revolution, which has endorsed her bid -- as has Sanders, who will campaign for her later this week -- were, King said, taking a breath to underline the understatement: "Very different."

When the rejiggered Pennsylvania map effectively pushed out her would-be primary opponent, but in the process left King in an even deeper red district, the prospect of meaningful aid from the party seemed that much further off.

"They don't seem to be too invested in races like this and I think that's part of the problem," King said, drawing a contrast with Our Revolution. "We need to show up in places that Trump won with the exclusive populism, the white nationalist populism -- we need to come in with an inclusive, welcoming populism, talking about the same things and inviting people into the process, because we can't afford to lose people to the Trump rhetoric in places like this."

Urbina, from Indivisible, echoed that argument, boosting a logic commonplace in the progressive activist trenches, and now -- if too slowly for some -- gaining currency with the party establishment, which has invested heavily, though more quietly, in what might have been dismissed as longshot races in the pre-Trump era.

"We've seen, even as we look at patterns emerging, there is a special interest (among Democrats) in deep red places that people might look over pretty quickly," she said, adding: "We're serious about building long-term power and not just transactional power."