Populism has spent years knocking on the door of the Republican Party. Next week, it may knock the door off its hinges—or burn down the house.

How did we get to the point where Donald Trump — barely a Republican — leads every poll in every early state and his most viable challenger is Ted Cruz, the most hated lawmaker among the GOP elite?

Republican Party leaders and elites brought about this moment by shooing away, squelching, or trying to cheaply co-opt the populist fervor that has been growing on the Right for years.

The GOP could have harnessed the emotion, and steered it towards conservative policy goals and Republican victories. But populism has always seemed uncouth or foreign to much of the GOP elite, and it could upset the donor base and the K Street class.

But the populism didn't go away, and on Monday across the prairies of Iowa, it could shake the Grand Old Party to its foundations, toppling conservatism, and erecting authoritarian identity politics in its place.

Populism is best understood not as an ideology, but as a sentiment, a fervor that waxes and wanes and changes shape.

In the days of Jimmy Carter's malaise, Ronald Reagan tapped into populism by stoking "deep-felt patriotism and nationalism," as Pat Buchanan, a Reagan-aide turned populist politician described it to me. Buchanan puts great weight on Reagan's 1978 debate against William F. Buckley over the Panama Canal Treaty — whether to cede it back to Panama or keep it. "I think that created a bond between him and middle America, which he nurtured. There was a sense not only of patriotism, but nationalism."

Buchanan backed Reagan over incumbent Ford in 1976, and over establishment favorites like Bob Dole and George H. W. Bush in 1980. Reagan in both elections saw the widespread discontent breeding a populist fervor, and was able to "Step out in front of this movement," Buchanan says, "and lead it and shape it and direct it."

Reagan promised an economy that would lift all boats, and toughness on crime — which at the time was rampant and worsening — and he did it with soaring, often populist rhetoric.

This explains the "Reagan Democrat" phenomenon. John F. Kennedy won 63 percent of Macomb County, Michigan, in 1960. Reagan won 66 percent in 1980.

Reagan welcomed populism aboard in 1980, and he neither ignored it nor put it in charge. Reagan governed as a conservative, cutting taxes and regulations, all the while keeping the populists at his side.

The 2008 bailouts sparked the latest populist fire. Managing it and shaping it was once again the Republicans' task. But nobody was up to the job.

By the second half of 2009, some in the GOP were warning of the dangers of populism. The National Republican Senatorial Committee went out of its way to prop up establishment candidates, officially backing Charlie Crist over Marco Rubio (lobbyist Bob Dole actually stayed with Crist after he bolted the party), and trying to find a challenger to Pat Toomey after he drove Arlen Specter to the Democrats.

Bailout lobbyists held fundraisers for Mitch McConnell's handpicked candidate Trey Grayson running against Rand Paul in Kentucky. Yet the party leadership was swamped by the Tea Party, losing Pennsylvania, Utah, Florida and Kentucky to the insurgents.

It was blindingly obvious by 8:20 p.m. Eastern Time on May 18, 2010 — when Grayson conceded to Paul — that all the energy in the GOP was populist.

George W. Bush was despised for his overspending and his bailouts. Barack Obama had gotten in bed with the special interests he ran against, pushing bailouts, a pork-laden stimulus and lobbyist-crafted health-care "reform." Obama spat on those bitterly clinging to guns and Bibles, and he promised to "remake" America.

Many GOP leaders missed the call. Others opposed it. "We don't need a lot of Jim DeMint disciples," Senate Majority Leader-turned-lobbyist Trent Lott told the Washington Post in July 2010. "As soon as they get here, we need to co-opt them."

The Tea Party carried Republicans to a House majority in 2010, but then the party clamped down on the populism. Notably, they pushed an immigration bill coveted by the business lobby.

In their approach to unseating Obama, the party rejected a populist angle, as well. Mitt Romney surrounded himself with a cadre of corporate lobbyists and leaned on Wall Street for fundraising, rather than on the huge grassroots donor network the Tea Party had uncovered.

Romney seemed to actively push away the working class, declaring the bottom 47 percent of income earners irresponsible welfare cases who belonged in the Democratic Party.

Why has the GOP behaved this way? Part of it is just old habits. Part of it is that populism would require war against corporate welfare and the lobbyist revolving door that enriches all of these insiders at the expense of the rest of the country.

But part of it is elite identity politics: Republican leaders would rather be the party of White-Bread suburbia than of Blue-Collar America. Republican leaders want to be the party of Scarsdale, New York, and so they ignore Upstate New York, where the voters are up for grabs.

So Donald Trump has occupied Upstate New York — and Western Pennsylvania, and Lowell, Massachusetts. And maybe Iowa and New Hampshire

"They left a vacuum," Buchanan says of today's GOP. "They left it vacant," and now "Trump has come in and he's walked off with Middle America."

The force of populism, recently in the GOP hands, is now far beyond their control.

House Speaker Paul Ryan agrees the party missed an opportunity. "What we lacked was imagination in an agenda," Ryan told me over the phone Wednesday. "People don't know whether we stand for them or not."

If Trump is reaching these voters with closed borders and protectionist policies, how can Ryan attract them while holding onto his free-enterprise, small-government beliefs?

One step, Ryan says, is battling against the rigged game. "When they see so much crony capitalism — when you have Big Business and Big Government making common cause and stacking the deck," struggling Americans "begin to lose faith in the system."

Ryan argues that good governance and limited government can tap into the populist sentiment by devolving power from the elites to the people.

Restoring Constitutional separation of powers and tax reform are great issues. But will anyone hear these arguments over Trump's loud gongs and clashing cymbals?

The populist fervor gave Republicans a chance in 2010 to become the party of Mike Lee and Rand Paul. They said no. In 2012 they perhaps could have embraced populism and become the party of Ted Cruz. They said no. Monday the GOP establishment may get steamrolled by populism and become the party of Donald Trump.

Timothy P. Carney, The Washington Examiner's senior political columnist, can be contacted at tcarney@washingtonexaminer.com. His column appears Tuesday and Thursday nights on washingtonexaminer.com.