Jeb Bush is maybe the weakest frontrunner for the Republican presidential nomination since Nelson Rockefeller. He's weaker even than Rudy Giuliani, who after a year atop the polls didn't win a single primary. And there is one issue that could easily be used to fell Bush in the primaries: immigration.

But that will be much easier said than done.

First, there aren't many candidates who are well positioned to do it. Scott Walker, Rick Perry, Marco Rubio, Rand Paul, Ben Carson and Mike Huckabee have all supported some kind of pathway to citizenship for illegal immigrants that many Republicans will see as amnesty.

Some have gone further out on the path-to-citizenship limb than others. (It will be easier for Walker to back away from his somewhat vague immigration comments than for Rubio to disavow his participation in Gang of Eight.) But like Lucy, they've all got some 'splaining to do.

That leaves Rick Santorum and Ted Cruz, who aren't polling well at the moment, as the candidates best situated to challenge Bush on the issue.

Second, it will be difficult to take a hard line on immigration and raise money. Republican donors want amnesty. They really want it.

Now, that hasn't been enough to get House Republicans to pass a bill to that effect. And there is also plenty of money a Republican with a different immigration opinion can raise from grassroots small donors. But it's an obstacle.

Third, it is easy for Republican candidates to say they are against amnesty while simultaneously supporting immigration policies many conservative activists regard as amnesty. President Obama's executive action makes this even easier — even Jeb Bush can say he opposes that.

While Republicans who oppose tax increases or abortion have pretty straightforward policy positions, conservative unease with current immigration policies is less detailed and harder to articulate.

Republican donors and strategists can easily massage these inchoate sentiments to produce poll results that seem to indicate GOP support for legalizing most illegal immigrants. But you'll notice that the politicians closest to the Republican electorate are least likely to behave as if they take these poll results seriously. That's for a simple reason: they don't.

Immigration poll responses tend to vary widely based on the wording. The polling that tends to produce the most pro-legalization results among Republicans assumes either that all the conditions for legalization will be met (everything from fines for individual amnesty applicants to a secure border) or that the only other alternative is mass deportations.

Here's a poll that throws a wrench into Republican machinations on this issue: Gallup found that 84 percent of Republicans are dissatisfied with current immigration levels, whether legal or illegal, and many of them want less of it.

Leading Republicans (with the exception of Santorum) like to say they are for legal immigration but against illegal immigration. How much of the conservative opposition to illegal immigration is shorthand for the belief that the country is receiving too many imigrants?

Some Republican voters may fear unskilled immigration will further tax means-tested government programs, because the immigrants are relatively poor even if they work hard. Others may worry about linguistic divisions and social cohesions. Still others may fear economic competition from immigrants, even of the higher-skilled variety.

Even many Republicans who feel this way have respect for the immigrants themselves and a concern about being perceived as racist. That's what makes the discussion of the issue through the prism of illegal immigration — a nation of immigrants and a nation of laws — more palatable.

Sen. Jeff Sessions (R-Ala.) is trying to help his party articulate a different immigration message. In his Immigration Handbook for the New Republican Majority, Sessions says, "I am opposed to any immigration policy which makes it harder for the unemployed to find jobs and easier for employers to keep pay low."

That's a message that doesn't bash immigrants, doesn't make distinctions based on race or ethnicity — and doesn't focus exclusively on illegal immigration at the expense of legal immigration.

The first Republican presidential candidate who picks up Sessions' framework will be in a good position to challenge Bush on immigration in a way that cannot be counteracted by playing word games over the meaning of amnesty.

Will any GOP aspirant decide they are up to the task?