Marco Rubio has a dubious distinction among the top-tier Republican presidential candidates. He’s the only one who crafted, sold and passed through the Senate a so-called comprehensive immigration reform that is anathema to the right.

As Rubio has demonstrated considerable political strength, the spotlight has turned to him. Inevitably, his role as frontman for the “Gang of Eight” bill will get extensively relitigated — and it should.

It was a colossal political and policy misjudgment. Among the flaws of the bill was the elemental one that it put an amnesty (sorry, that’s exactly what it was) before enforcement. Rubio got a respectful hearing from conservative talk radio. He and his team aggressively rebutted critics. And the bill passed with 68 votes in the Senate, enough, it was thought at the time, to bulldoze the opposition in the House.

Instead, House conservatives dug in, and eventually, Rubio declared his own handiwork a mistake.

It’s a hell of a mulligan, and there is, understandably, lingering distrust. House Speaker Paul Ryan is a Kempian true believer in a latitudinarian immigration policy. If you couple him with a President Rubio, the erstwhile champion of a sweeping amnesty and large-scale increase in immigration, they could be the Dynamic Duo of everything grass-roots conservatives oppose on immigration.

The reassurances from the two aren’t always very reassuring. Sometimes, Ryan, who has pledged not to move a comprehensive bill during the Obama administration, sounds as if he is implicitly saying, It’s a real shame that Barack Obama is president since we can’t pass a sprawling, deceptive, impossible-to-administer 1,000-page immigration bill. But don’t worry. Once there’s a Republican president, we’ll really get after it!

Rubio often sounds more categorical when explaining that immigration reform has to be incremental, not comprehensive. But conservatives, as Mark Krikorian of the Center for Immigration Studies argues, need to push to nail him down with specific promises that would exact a real political price if he ever backtracked on them.

What does it mean that enforcement will come first, as Rubio says? If it is only a promise to pass enforcement legislation before moving with dispatch to pass the other constituent parts of so-called comprehensive immigration reform, it is a meaningless commitment to a particular parliamentary path to the same end.

Enforcement-first must have some unmistakable content. It should require that an E-Verify system is fully functioning, which means that it must withstand the inevitable legal challenges from the ACLU and ethnic pressure groups and that all employers are enrolled, a years-long undertaking.

It should require that an entry-exit system is up and running and tracking 100 percent of people coming here by sea or air (tracking people coming by car is much harder, but there should be a benchmark for that, too).

It should require a working system of cooperation between the federal government and local police.

It should require prosecution of all illegal entries, a formality but one that sends a clear signal that we take the offense seriously (the Bush administration had begun doing this in certain sectors of the border, but the Obama administration, of course, gutted the effort).

These systems should have to show results, say, a year-over-year decline in the illegal-immigrant population over the course of three years. If Rubio is serious when he says, as he did the other day in New Hampshire, that his first priority is “illegal immigration being brought under control,” none of this should be a problem for him.

Rubio says his second step on immigration would be to modernize the legal system to emphasize skills. This shouldn’t be controversial, but he said the same thing during the Gang of Eight debate, even though the bill would have welcomed more unskilled immigrants and increased overall levels of legal immigration considerably.

This is an issue that has now developed beyond the typical lazy distinction Republicans make between illegal immigration (inherently bad) and legal immigration (inherently good). Even Donald Trump falls into this when he says his wall is going to have a great big door, and Ted Cruz repeatedly talked of how fervently he supports legal immigration during the Gang of Eight debate (he tried to amend the bill to increase legal immigration even more).

Rubio should promise that any change in the criteria for legal immigration come in the context of an appreciable drop in overall immigration levels. Not only has legal immigration been running at historic highs for decades now, Republicans strongly back reducing it, according to a recent Pew survey. It found that 67 percent want to reduce immigration and only 7 percent to increase it.

If Rubio’s increased high-skilled-immigration is merely layered on top of current levels, it will represent a continuation of the Beltway’s default toward more immigration no matter what. And it will continue to orphan all those Republicans who feel as though no one represents their views, except perhaps Trump, and even he can’t always keep his story straight.

Conservatives will want to hear more from Rubio — on Obama’s executive amnesty, on guest workers, on the pathway to citizenship — but making these two assurances wouldn’t contradict anything Rubio has said over the past year, and it would at least alleviate concern that his new approach is boob bait for Bubba in the GOP primaries.

But the doubts will never go away, nor should they. On immigration, the lesson from decades of cant and false promises by both parties is clear. With apologies to Ronald Reagan, it is simply, “Don’t trust.”