For months now, even as Donald Trump has maintained a strong lead both in the national polls and in most of the early-state polls, a wide range of journalists and analysts have confidently assured us that Trump is not the “real” front-runner in the GOP presidential contest.

There are fewer people offering those assurances today, and the ones still making them seem far less confident. It’s getting harder and harder to deny that the billionaire Manhattanite remains the man to beat.

How did that happen?

For all the disparate theories that pundits have put forth, Occam’s razor tells us that the simplest explanation is probably closest to the truth. In that spirit, I tend to agree with former Mitt Romney adviser Eric Fehrnstrom: "The key to understanding the contest for the Republican nomination is immigration. Once you have the key, you can unlock all the mysteries of the race."

The Republican Party started out encouraging candidates to support comprehensive immigration reform, a massive error in political judgment. Trump went in the opposite direction and has been rewarded with a commanding lead.

Indeed, it’s truly remarkable how much Trump has changed the immigration debate among GOP presidential hopefuls.

Just look at Ted Cruz. Before Trump came along, the Texas senator was on record advocating a fivefold increase in the cap on H-1B guest-worker visas, saying it would “encourage economic growth and create new jobs in America.”

Yet the immigration plan that Cruz released in mid-November would “suspend the issuance of all H-1B visas for 180 days to complete a comprehensive investigation and audit of pervasive allegations of abuse of the program.”

It would also tighten H-1B requirements and strengthen protections for American workers, while implementing a bevy of border-security and internal-enforcement measures, abolishing birthright citizenship, scrapping the so-called diversity visa, ending chain migration and preventing “any increases in legal immigration so long as American unemployment remains unacceptably high.”

Now Trump is not the only reason that Cruz toughened his stance on immigration, but there’s no question that the real estate mogul has produced a significant shift in how Republican candidates perceive and/or approach the issue. One need not be a Trump supporter to recognize his impact.

How will GOP congressional leaders respond? That remains to be seen. As my former Hudson Institute colleague John O’Sullivan has written: "[Trump] has realized that there is a great opportunity for a political entrepreneur in a two-party system when one party systematically betrays its supporters as the GOP establishment has done on the issue of immigration."

Others may have had the same realization but lacked the means to do anything about it. Trump has the money and the bombast to fight. He has gained within the GOP the mass support of alienated conservatives that has clustered around insurgent parties in other countries.

Making concessions to Trump in order to undercut him would therefore seem to be the order of the day. Instead, the House Republican majority negotiated with President Obama on a budget package that was seemingly designed to outrage Republicans outside Washington—everything from sanctuary cities to the quadrupling of temporary work visas.

The enormous H-2B visa expansion was not an essential component of the budget deal. That is to say, adding it to the final package was not necessary to secure Democratic support and prevent a government shutdown.

But GOP leaders added it anyway, which confirmed that their priorities on immigration are still very different from the priorities of most Republican voters. “The tone-deafness of such a move in the midst of the Trump surge was simply breathtaking,” wrote Hoover Institution scholar Jeremy Carl.

A Pew Research Center survey conducted earlier this year found that 67 percent of Republicans think immigration levels should be reduced, whereas only 7 percent think they should be increased.

If GOP leaders had better understood and/or taken seriously their voters’ legitimate concerns about immigration, the Trump surge might never have happened. But they didn’t, and it did.

Rachel Currie is a senior fellow at the Independent Women’s Forum. She was chief speechwriter for Republican Senator Jon Kyl and managing editor of the Hudson Institute.