Sen. John McCain (R-Ariz.) heads to the stage in Phoenix after being declared the winner in the Arizona Republican primary on Aug. 30. (Ross D. Franklin/AP)

Senate Republicans, for the second straight election cycle, have surrendered none of their primary fights to conservative challengers.

In doing so, they left conservative hard-liners without a single banner victory to claim as their own in recent years. And Sen. Majority Leader Mitch McConnell (R-Ky.) increased his chances of keeping the Senate in Republican hands in 2017 by ensuring that his crop of nominees is more palatable to general-election voters.

The handy victories by Sens. John McCain (R-Ariz.) and Marco Rubio (R-Fla.) in primaries Tuesday completed the sweep for the Republican establishment, duplicating a feat McConnell first set as a goal during the 2014 election season.

The dominance by Senate Republicans could provide the road map for GOP strategists in other races, particularly future presidential campaigns. It could also mean the overall weakening of the conservative movement’s ability to raise money and pressure senators to vote their way for fear they will draw a strong primary threat.

Time and again, Senate Republicans have quashed challenges from fringe conservatives — candidates who on paper sound a lot like Donald Trump, who overwhelmed establishment candidates in the presidential primary but has struggled to expand his message and gain traction in the general election against Hillary Clinton.

After seeing three senators fall during primaries in 2010 and 2012 and insurgents knock off establishment-backed Republicans in key seats, GOP strategists adopted an overwhelming-force doctrine for handling primary challenges. They leveraged their financial edge and set out to define the opponents early, before they could gain momentum among the conservative grass roots, using modern technology and also spending heavily on positive TV ads to improve the incumbent’s standing among conservative voters.

That strategy was considered essential in protecting GOP incumbents such as Sens. Pat Roberts of Kansas and Lamar Alexander of Tennessee in the 2014 midterms.

“The culture of primaries within the Senate GOP campaign apparatus is where the entire party needs to go,” said Josh Holmes, who was McConnell’s 2014 campaign manager and whose firm, Cavalry, worked for McCain and several Republican incumbents. “If your job is to win elections, you don’t just throw up your hands and say, ‘Gosh, I sure hope voters don’t pick the guy who can’t possibly win.’ ”

Sen. Kelly Ayotte (N.H.) is the last Republican facing a primary this cycle, on Sept. 13, but she is facing only token opposition and all sides expect her to win the nomination.

The old sports adage that winning begets winning, and the corollary that losing begets losing, might apply here. In early 2014, a collection of Washington-based groups with ties to tea party activists were on the march, helping challengers in a clutch of primaries, including in a challenge to McConnell.

Now, after losing big in 2014 and barely engaging in 2016, those groups are a shell of their former selves.

Take Senate Conservatives Fund, a group originally founded by Jim DeMint, the former senator from South Carolina who is the political godfather to the Republican anti-establishment movement.

In the 2012 elections, SCF raised $16 million, $6.5 million of which was steered directly into the coffers of the most-conservative candidates. An additional $3.5 million went to its own independent campaigns, including TV and radio ads benefiting those conservative challengers. That year, SCF helped knock off Sen. Richard Lugar (R-Ind.), a 36-year veteran, and helped elect Sen. Ted Cruz (R-Tex.) over an establishment Republican.

In the 2014 elections, SCF raised $16.6 million, directing $5 million in contributions to candidates and spending $3.6 million on its independent campaign to benefit conservatives. This year, however, SCF’s finances crumbled — as of June 30, the group had raised just $4 million and spent less than $600,000 on its own campaign for conservatives.

With so little cash, the group focused on direct mail instead of broadcast ads, its largest expenditure being $65,000 in mailings to boost Darryl Glenn, a conservative who won the Colorado primary. Glenn trails Sen. Michael Bennet (D) badly and has been left for dead, politically speaking, by national Republicans.

SCF’s staff declined requests for comment this week about the group’s plans.

What changed, according to Holmes and other GOP establishment operatives, was the declaration from McConnell that winning the general election was the only thing mattered. “I think we are going to crush them everywhere,” McConnell boasted to Carl Hulse of the New York Times in March 2014.

The establishment didn’t exactly crush those conservatives everywhere. Aside from McConnell’s primary win by 30 percentage points, the GOP incumbents often won in ugly fashion, but establishment Republicans took every race of consequence in 2014.

The ultimate win for McConnell came in Mississippi, where Sen. Thad Cochran’s narrow runoff win, boosted by African Americans crossing into the GOP fold, deflated conservatives who’d rallied around Chris McDaniel, a state senator.

The 2016 field looked moderately promising for insurgents, given it included a trio of octogenarians with at least six Senate terms under their belt. Also, the retirement of Sen. Dan Coats (R) set up an ideological clash in the Indiana primary, and before Rubio reversed course to run for another term, the GOP race in Florida was fertile ground.

All those races fizzled for conservatives.

Their finances fell to a fraction of past insurgent campaigns’, and the candidate quality was subpar. Rep. Marlin Stutzman, running in Indiana, went through multiple layers of campaign staff. Stutzman received modest help from SCF and the Club for Growth, a conservative group that remains well-stocked financially, while Rep. Todd Young’s campaign had millions of dollars in support from establishment-friendly super PACs. Young won convincingly.

[John McCain is in the fight of his political life in the age of Donald Trump]

In Arizona, Kelli Ward, 47, presented quite a contrast to McCain, 80. She is young but has experience in the state Senate and is a conservative who likes Trump and Cruz. Those two, who have both feuded publicly with McCain, combined for more than 70 percent of Arizona’s votes in its March presidential primary.

But McCain and his allies went to work early on Ward’s positions, trying to define her as a nonconservative with strange views. They highlighted her town hall on “chemtrails,” a conspiracy theory that airplanes spray the skies to manipulate weather and even control human minds.

McCain’s team used Internet searches to distract from Ward’s message. Anyone in Arizona who searched “Kelli Ward” on Google or Bing found a website called “What Kelli Ward Says” as the first click. It was filled with negative items paid for by McCain’s campaign.

“The establishment has won this battle by spending untold millions to call me liberal, dangerous and weak,” Ward said Tuesday night in her concession speech, adding, “I hope the senator can rest comfortably with his conscience.”

It wasn’t pretty, but McCain won and Republicans have their best chance at keeping the seat in the red column in November. It is a different story at the top of the ticket, and many establishment Republicans look at these Senate races as the petri dish for how to get a more palatable candidate in the next presidential primary.

Correction: An earlier version of this post misidentified the Republican who won the Colorado primary.