Ben Carson just became arguably the most liberal candidate in the GOP race on immigration

Ben Carson wants unauthorized immigrants currently in the US to get legal status.

That's what he said at an event in Virginia Wednesday, recorded by Fox News:

.@RealBenCarson on undocumented immigrants: "I propose that we give them a 6 month period in which to register..." https://t.co/sUxRQoWmbs

— Fox News (@FoxNews) November 11, 2015

Breaking this down, there are three elements to Carson's plan:

• Unauthorized immigrants currently in the US would have six months to register. Those who didn't register would be "treated as criminals," i.e., probably deported (since that's what happens to many unauthorized immigrants checked into jails right now).
• The unauthorized immigrants who registered and had a "pristine record" would be eligible for legal status. (Carson describes this as "guest worker" status.)
• Just being a guest worker wouldn't make someone eligible for citizenship, but if legalized immigrants wanted to apply for citizenship they could go about it through the existing process.

This is a big opportunity — and a big test — for the anti-amnesty right

To be clear, this is definitely to the left of what Marco Rubio has been saying (Rubio's way of avoiding the immigration issue is to say that Congress shouldn't even talk about what to do with unauthorized immigrants until the border is secure). And you could make a convincing argument that, at least as this clip presents it, Carson is to the left of Jeb Bush on the issue (Bush has wavered on whether immigrants who are currently unauthorized would ever be able to become citizens of the US).

Many grassroots conservatives distrust Bush and Rubio because of their immigration records. Ben Carson, on the other hand, is leading in several national polls — because of his strength among grassroots conservatives. He's become the favorite among evangelical voters. He's wrestling with Donald Trump for command of the "outsider" lane in the Republican primary (with Ted Cruz not so secretly hoping the two of them knock each other out and leave him to step into the role).

Donald Trump's rise was fueled in large part by his ability to tap into anti-immigration sentiment among a segment of the Republican base. The question is how big that segment really is. If immigration — and more specifically opposition to "amnesty" — is a motivating issue for Republican base voters in general, Carson's support should take a big hit for this. If this doesn't damage him, it will be evidence that the voters for whom immigration is a key motivating issue are a relatively small bloc within the party — and a bloc Trump has already locked up.

What to look for Carson to say next

Because this is a minute-long video clip, not a white paper, there are plenty of open questions about what Carson's plan would actually be. Some of those are wonky questions that aren't likely to come up in a presidential campaign, like, "How often would people have to renew guest worker status, or would they have to leave after a certain amount of time?"

But some of the question marks present opportunities for Carson to caveat or neuter his position if he worries he's gone too far — or to lean in, if he's decided this is an issue on which he wants to distinguish himself from Trump.

• "Only after the border is secure." This is the most likely move for Carson to make, and it's a common rhetorical tic among Republicans and Democrats alike. (The difference is that most Democrats, and some Republicans, are okay with a bill that sets a timeline for securing the border and guarantees immigrants will be legalized at some point during that timeline.) The genius of it is that no one can agree on what a secure border looks like, but everyone knows we don't have it yet — so this makes it unlikely a President Carson would ever have to sign an "amnesty" bill.
• "When I say pristine, I mean..." Carson only wants to legalize immigrants with a "pristine" record. If that means that immigrants can't have committed any felonies, that's a relatively generous standard. But there are plenty of ways that politicians can shrink the population of people eligible for legalization until it's basically nonexistent: requiring a certain level of education, a certain level of income, etc.
• "They would have to return and come back." Carson is currently saying that for legalized immigrants to become citizens, they'd have to go through the current process for applying for citizenship. He says that part of that is leaving the country and coming back. (He happens to be wrong about that; some of the unauthorized immigrants currently in the US would be able to apply for green cards through relatives once they stopped being unauthorized, and in some of those cases they'd be able to apply for waivers of the three- and 10-year "bars" to reentering the country.) If he switches the order of that — and starts requiring immigrants to leave the country in order to register or get legalized to begin with — his plan would start to look a lot more like Donald Trump's. Requiring immigrants to leave the US when there's only a chance they'd be able to return is usually considered an offer few immigrants are likely to take, so in terms of legalization it would be pretty paltry.