It is hard to exaggerate how unlikely the victory was and what a shock it has been for the Washington establishment of the Republican Party, which had been celebrating what it thought was the end of the far-right grass-roots insurgency.

Even as voting continued the Washington Post reported that the only question about Cantor’s race yet to be answered was by what margin he would win.

Cantor’s internal polling had him 34 points ahead of the unknown who in the end beat him by 10 points, an economics professor named Dave Brat with an affinity for Ayn Rand.

No sitting majority leader had ever lost their seat in the primary, and though Cantor was not warmly viewed by his colleagues he was seen as the natural pick to take over from the House Speaker, John Boehner, when he chose to move on.

Cantor, a representative of Virginia, was one of the most recognisable political faces in the country, while Brat was unknown outside his district.

While Brat spent a measly $US123,000 ($131,000) on his entire campaign, Cantor shelled out $US168,000 for three fundraisers in Virginia steak houses.

Larry Sabato, one of America’s keenest political analysts and director of the University of Virginia’s Centre for Politics, was blindsided by the result.

“Everybody was,” he told Fairfax Media. “The only people who predicted this are liars.”

The news caused excitement throughout DC and was thrashed out at the table at Ambassador Kim Beazley’s residence where the PM dined with Republican guests.

A cheerfully mean-spirited piece on Cantor’s allegedly universal unpopularity in New York Magazine reported that “Anywhere professional politicians gathered [on Tuesday night], the Cantor news led to spontaneous, Berlin Wall-type rejoicing.”

Well, perhaps, but by Wednesday many Republicans, even those that didn’t particularly like Cantor and his leadership, were discussing the result as a catastrophe. The most pessimistic of them see in Cantor’s loss evidence of a rot in the Grand Old Party that, left unchecked, could  permanently relegate it to a minority.

Shattered by their loss in 2012, establishment Republicans ceased to see the Tea Party movement that erupted in the wake of the financial crisis and Obama’s first victory as useful allies. While the group’s energy had helped Republicans take over the House of Representatives in the 2010 mid-term election it soon became clear that the real targets of the movement were not just Democrats but Republicans who refused to capitulate to its demands.

Worse, many of those demands were too radical for middle America. The Tea Party was knocking off viable Republican candidates or incumbents and replacing them with ideologically pure and electorally abysmal anti-government activists.

Radical Tea Party candidates cost Republican Party key seats in elections in 2010 and 2012. Among them were Todd Akin who, days before the 2012 election, said women rarely become pregnant due to “legitimate rape”; Richard Mourdock who agreed with him; Sharron Angle who said Tea Party losses may provoke armed insurrection and opposed abortion in cases of rape and incest;and Christine O’Donnell who perplexingly denied being a witch.

In Congress the Tea Party was not just making life a misery for Democrats but for Republicans who wanted to do business with Democrats and get some governing done. In line with the maxim of anti-tax advocate Grover Norquist that “bipartisanship is date rape” the Tea Party railed against cutting any deals with the White House or with Democrats who hold the balance of power in the Senate.

This has forced Obama to use executive action rather than work with Congress, meaning Republicans had no say in the construction of, for example, the tough new carbon emission regulations announced last week.

In the Republican-dominated House of Representatives Tea Party activists led the disastrous debt-limit brinksmanship, seeking to block the American government’s authority to pay its own bills. The move damaged America’s global standing and its credit rating, appalled Republican allies in business and finance and ended in a humiliating backdown.

This catastrophe finally prompted establishment Republicans include House Speaker John Boehner and Senate minority leader Mitch McConnell to seek to exert some authority over the radicals and chart a course back to more temperate politics.

For a while it seemed to be working. With an eye on the 2016 presidential election the GOP’s governing body began cutting the number of planned candidate debates – which tend to favour Tea Party outsiders – and bring forward the primaries in more moderate states to give mainstream candidates a chance to build momentum.

Throughout this primary season for the November mid-term elections Republican establishment figures, such as South Carolina’s Lindsey Graham, had been successfully fighting off Tea Party attacks. Even as counting went on in Cantor’s district eulogies for the Tea Party were being written by DC political analysts.

Then came the Cantor upset and Tea Party celebrations throughout the country.

To establishment Republicans, Brat’s victory shows that the Tea Party insurgency has yet to be broken. Worse it suggests that many Republican candidates who might have been able to moderate their stances on key topics to win seats in the 2016 election might now have to veer dangerously right to fight off Tea Party challenges in primaries.

But the true horror for the centrists is the line of attack Brat used to destroy Cantor – immigration reform. Cantor had begun negotiating with Democrats to pass a reform bill that would allow many of the 11 million undocumented immigrants in the nation to work towards citizenship. Brat picked this up and attacked it mercilessly. Though he had no money for advertising, right-wing radio hosts championed his anti-immigration stance. In the end Cantor distanced himself from immigration reform as well, but it was too late.

Republican strategists in DC fear that, unless the party backs immigration reform, it can never win the support of America’s Hispanic population, the fastest-growing in the nation, and will not be able to take back the White House as a result.

Sabato believes such fears are overblown and that the immigration deal was already dead, but that is little comfort to fretting Republicans.

After Cantor’s defeat one immigration-reform-supporting conservative emailed The Atlantic’s Molly Ball, telling her: "I can't vote for Democrats because I am pro-life, but my party seems beyond repair."

David Frum, a commentator and former staffer to George W. Bush, wrote in the same magazine that if the Republicans surrender to the Tea Party, “they condemn the whole conservative project in America to the destructive leadership of fanatics.

“If Republicans don’t develop [leadership] soon, we might just as well already rename our dysfunctional party the Committee to Elect Hillary Clinton.”