A pro-Trump outside group aligned with Stephen Bannon endorsed three Republicans running for Senate on Wednesday, firing the starting pistol on what's shaping up to be a nasty primary season for the GOP.

The Great America Alliance — an antiestablishment group which counts Bannon’s political adviser Andy Surabian as a strategist — announced it will support West Virginia Attorney General Patrick Morrisey (R), Montana state Auditor Matt Rosendale (R) and Rep. Marsha Blackburn Marsha BlackburnDemocrats smell blood with new DHS whistleblower complaint Hillicon Valley: Election officials prepare for new Russian interference battle | 'Markeyverse' of online fans helps take down a Kennedy | GOP senators unveil bill to update tech liability protections GOP senators unveil new bill to update tech liability protections MORE (R-Tenn.) in their bids for Senate.

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Sources familiar with the decision told The Hill that Bannon, the Breitbart chairman and former White House chief strategist, is asking the candidates he backs to oppose Sen. Mitch McConnell Addison (Mitch) Mitchell McConnellMomentum growing among Republicans for Supreme Court vote before Election Day Trump expects to nominate woman to replace Ginsburg next week Video of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral MORE (R-Ky.) as the next majority leader, although it’s unclear if they've all committed to this. Rosendale has said he will support McConnell.

Bannon is also asking candidates to support eliminating the legislative filibuster in the Senate and to back President Trump’s trade and immigration policies.

"Establishment forces on both sides of the aisle in Washington have either been unwilling or unable to effectively advance President Trump's agenda, which is totally unacceptable," said Great America Alliance co-chairman Eric Beach. "The best way to advance the 'America First' movement is to hold elected leaders accountable — get on board and get the job done or be replaced by someone who will."

Bannon has said he intends to support primary challengers running against every Republican incumbent, with the exception of Sen. Ted Cruz Rafael (Ted) Edward CruzVideo of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral Sunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Democrat on Graham video urging people to 'use my words against me': 'Done' MORE (R-Texas).

While Bannon and his allies are already tacitly supporting primary challengers to Sens. Jeff Flake Jeffrey (Jeff) Lane FlakeJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden Maybe they just don't like cowboys: The president is successful, some just don't like his style MORE (R-Ariz.) and Dean Heller Dean Arthur HellerOn The Trail: Democrats plan to hammer Trump on Social Security, Medicare Lobbying World Democrats spend big to put Senate in play MORE (R-Nev.), the first wave of endorsements focuses on Republicans running for open seats or ones currently held by Democrats.

Morrisey, who is vying for the nomination against Rep. Evan Jenkins (R-W.Va.) for the right to take on Sen. Joe Manchin Joseph (Joe) ManchinMomentum growing among Republicans for Supreme Court vote before Election Day Gardner on court vacancy: Country needs to mourn Ginsburg 'before the politics begin' Barrett seen as a front-runner for Trump Supreme Court pick MORE (D-W.Va.), made his name spearheading legal challenges against the Obama administration's environmental regulations.

Rosendale, who enters a crowded GOP primary in Montana for the right to challenge Sen. Jon Tester Jonathan (Jon) TesterPence seeks to boost Daines in critical Montana Senate race This World Suicide Prevention Day, let's recommit to protecting the lives of our veterans Filibuster fight looms if Democrats retake Senate MORE (D-Mont.), has crossover appeal to the GOP establishment.

Blackburn, who is running to replace retiring Sen. Bob Corker Robert (Bob) Phillips CorkerHas Congress captured Russia policy? Tennessee primary battle turns nasty for Republicans Cheney clashes with Trump MORE (R-Tenn.), could also attract support from Washington Republicans.

So far, Blackburn has steered clear of the nasty public feud between Corker and President Trump that exploded in recent days and further exposed the deep rift between establishment Republicans and grass-roots conservatives that will play out in primary elections across the country in 2018.

More endorsements, some for candidates who are challenging incumbent Republicans, are expected in the coming weeks.

Bannon and his allies are plotting an ambitious primary strategy in which no Republican, except for Cruz, appears safe. Great America Alliance’s allies have said they’re recruiting primary challengers for Sens. John Barrasso John Anthony BarrassoOVERNIGHT ENERGY: Democrats push resolution to battle climate change, sluggish economy and racial injustice | Senators reach compromise on greenhouse gas amendment stalling energy bill | Trump courts Florida voters with offshore drilling moratorium Senators reach compromise on greenhouse gas amendment stalling bipartisan energy bill The Hill's Morning Report - Sponsored by National Industries for the Blind - Trump, Biden battle over vaccine, economy; Congress returns MORE (R-Wyo.), Deb Fischer Debra (Deb) Strobel FischerHillicon Valley: Election officials prepare for new Russian interference battle | 'Markeyverse' of online fans helps take down a Kennedy | GOP senators unveil bill to update tech liability protections Google, Apple, eBay to meet virtually with lawmakers for tech group's annual fly-in Congress botched the CFPB's leadership — here's how to fix it MORE (R-Neb.) and Roger Wicker Roger Frederick WickerHillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll Shakespeare Theatre Company goes virtual for 'Will on the Hill...or Won't They?' Republican Senators raise concerns over Oracle-TikTok deal MORE (R-Miss.), who have not been openly critical of Trump, among others.

“They have to understand something — just voting is not good enough,” Bannon said Monday night on Fox News's "Hannity"’ “You have to have a sense of urgency. Nobody's safe. We're coming after all of them. And we're going to win.”

Republicans have a slim 52-48 majority in the Senate. The election map is favorable for the GOP in 2018, with Democrats defending 25 seats, including 10 in states that Trump carried in the last election.

But Republicans are beginning to worry about the damage their candidates could sustain, as well as the resources they’ll have to spend to make it through the primary season.

There are also fears that the GOP could squander seats that should be easy to defend by nominating weak insurgent candidates who lack general election appeal, as happened in 2010.

Some Republicans point to former Alabama state Supreme Court Justice Roy Moore (R), who trounced Sen. Luther Strange Luther Johnson StrangeSessions hits back at Trump days ahead of Alabama Senate runoff The biggest political upsets of the decade State 'certificate of need' laws need to go MORE (R-Ala.) in a special election primary last month with the help of Bannon and his allies, as a case study here.

Polls show Moore, who has come under fire for controversial remarks about race and his views on same-sex marriage, has only a single-digit lead in the polls over Democrat Doug Jones in deep red Alabama.

Bannon insists the so-called Tea Party problem will not be an issue this go-round.

“We're spending a lot of time with the grass-roots organizations to make sure that these candidates are fully vetted,” Bannon told Hannity. “You will see people announce this week with experience in government, you’re going to have some outsiders that are authentic, these people are real. It’s not like 2010. 2010 was the beginning of the Tea Party and it was just getting going. You will see real candidates and they will take on incumbents in every state and the Democrats after that.”

Bannon has also been meeting with donors and claims that his slate of candidates will be able to compete financially with well-funded incumbents. Moore was able to ride grass-roots energy to a victory despite being dramatically outspent by Strange, who benefited from millions of dollars from the Senate Leadership Fund, which is run by allies of McConnell.

“The donors are coming to us because they're tired of having their money burned up by trying to destroy people like Judge Moore,” Bannon said. “It’s a new game in town. We're going to cut off the oxygen to Mitch McConnell. Mitch McConnell's biggest asset is the money. We're going to make it the biggest liability. We're going after them tooth and nail.”

The Senate Leadership Fund declined to comment for this story.

But national Republicans are doubtful the Breitbart wing will be able to compete financially and believe their influence in the primaries is being overstated.

Many Republicans believe Bannon and his allies swooped in late on Moore’s behalf in Alabama and capitalized on a foregone conclusion.

“If you look at Alabama — Bannon came in a few weeks out and held a rally, that’s it,” said one Republican. “There was no ground game, no infrastructure, just the press ready to give him more credit than he deserved.”

Others note that beyond the primary challenges to incumbents, there are candidates who will get support from national Republicans and the Bannon wing alike, such as Rosendale, Blackburn and Ohio treasurer Josh Mandel, among others.

“If you look at the Republican Party as a whole, primaries are not new to anyone,” the Republican said. “After Eric Cantor Eric Ivan CantorThe Hill's Campaign Report: Florida hangs in the balance Eric Cantor teams up with former rival Dave Brat in supporting GOP candidate in former district Bottom line MORE, everyone has learned the importance of taking races seriously from day one, and that’s what every incumbent from House to the Senate is doing. Not having a primary in some of these cases would be great, but that’s not the reality of Republican politics.”