How big is immigration in the Republican presidential race? Most GOP voters name either national security or the economy as their most important issue, while a smaller number name immigration. But for a significant part of the Republican base, immigration is at least a near-the-top issue. And some observers, like Eric Fehrnstrom, a senior strategist for Mitt Romney who is unaffiliated in this year’s campaign, believe immigration is “the key to understanding the Republican race.”

That is why the two candidates most closely associated with immigration reform, Marco Rubio and Jeb Bush, are still, at this late date, struggling to reconcile their advocacy of comprehensive reform with the Republican primary electorate’s opposition.