President Obama on Thursday announced his much-anticipated executive action on immigration, and in doing so he lobbed a grenade squarely into the 2016 Republican primary race.

Obama's order will reshape how the feds prioritize deportations of undocumented workers, shielding an estimated five million of them from being kicked out of the country. "We shall not oppress a stranger for we know the heart of a stranger — we were strangers once, too," Obama said, quoting scripture.

By moving ahead solo before the new Congress is sworn in, Obama ensured Republicans will finally have to address immigration reform next year — and on into 2016. This poses a unique problem for two of the GOP's biggest potential presidential candidates who have broken with the party on the issue.

Former Florida Gov. Jeb Bush upset the rank and file earlier this year by calling for a compassionate approach to immigration. In April, he said undocumented workers who enter the country illegally do so as "an act of love" because they "are actually coming to this country to provide for their families." Then there's New Jersey Gov. Chris Christie, who in January broke with the national party and signed a state-level version of the DREAM Act. Like Bush, he framed his support for reform through the lens of strengthening families.

Among the general public, neither position is politically abhorrent. Polls have consistently found robust majorities of Americans in favor of a pathway to citizenship for undocumented workers. And Americans overwhelmingly supported the Senate's sweeping immigration bill, which incorporated elements of a proposed federal DREAM Act.

Still, both candidates may never reach the general election because their positions are anathema to Republican primary voters.

To understand this predicament, it's worth revisiting Texas Gov. Rick Perry's (R) short-lived 2012 campaign. Though Perry flopped mainly because he displayed the brains and demeanor of a limp windsock, his signature on a bill granting in-state tuition to illegal immigrants left him vulnerable on the right. In one memorable debate, Perry's opponents formed an unspoken alliance and took turns whooping him over that law. Even the humorless Mitt Romney snuck in some jabs, and Perry crumbled.

Little has changed since then. Though Republicans admitted after the 2012 election they needed to do a better job appealing to Latino voters, they saw no viable way to do so without pissing off a base that seethes at even a whiff of "amnesty." The howling last year from conservatives displeased with the Senate's bipartisan bill — a bill supported by more than 70 percent of Latino voters and a huge majority of Independents, too — was so loud it forced cowering House Republicans to stymie the measure into oblivion without even giving it a vote.

Before Thursday, immigration reform was on course to play a role in the next GOP primary. Yet with one stroke of his pen, Obama elevated it from a secondary issue to the very forefront of national debate. Christie and Bush, who already had a difficult messaging task ahead of them, must now either flip the bird to GOP voters or tie themselves in knots trying to walk back their previous positions. (Christie is already engaging in some preemptive flip-flopping, saying he has a top secret position on immigration he can't reveal unless and until he runs for president.)

Even before the president announced the specifics of his plan, Republicans were calling him a tyrant, a dictator, and a monarch. The most unhinged among them suggested responding with another government shutdown or, perhaps, impeachment. Spurred on by grassroots fury and a browbeaten GOP leadership, the wild-eyed vitriol will only deteriorate further into self-parody now that Obama has acted. Come 2016, you can easily imagine Republican presidential candidates stumbling over each other to prove who among them will build the biggest danged fence.

A fence-measuring litmus test on immigration won't favor Bush and Christie given their previous refutations of party orthodoxy. By contrast, Perry and Wisconsin Gov. Scott Walker, both potential 2016 candidates themselves, are gleefully threatening to sue the president for his alleged constitutional encroachment.

How can Christie and Bush compete with that?