We keep hearing that in order for immigration reform to pass Congress, it will have to be pushed considerably to the right. But given the current Gang of Eight compromise’s heavy emphasis on border security — combined with a path to citizenship that now stands at 13 years — how much more conservative can the measure actually become? And what would that look like, anyway?

Today we got the first glimpse of an answer to these questions, as the Senate Judiciary Committee took its first stab at marking up the bill. And the answer is pretty clear: While there may be some room for movement, the bottom line is that this bill just can’t be made much more conservative without undermining the very things that make it amount to comprehensive reform.

There were two main votes today that underscored this reality. First, the Judiciary Committee rejected a Chuck Grassley amendment that would have required the border be secure for six months before the 11 million get legal status. This would have struck directly at the bill’s core — requiring a trigger for legalization to occur later, rather than setting legalization in motion while requiring various security benchmarks to be hit. The fact that two Republicans — Lindsey Graham and Jeff Flake, both members of the Gang of Eight — voted this down shows that there is bipartisan support for rebuffing the right’s most ambitious efforts to undermine reform.

Meanwhile, four Republicans (Graham, Flake, Orrin Hatch, and John Cornyn) voted with Dems to pass what’s called a “substitute amendment,” i.e., a new version of reform tweaked to deal with various problems — which signals strong bipartisan support for the compromise. The four Republicans who voted against this — Grassley, Ted Cruz, Jeff Sessions, and Mike Lee — illustrate that a hard right bloc remains opposed at all costs to reform that includes a path to citizenship, but that they were unable to derail the bill so far.

A number of security-oriented Republican amendments to the bill did pass, which Dems, and pro-reform Republicans, will point to as proof that they’re willing to accommodate conservative objections. Marco Rubio — who is trying to win conservatives — hailed them as significant today. But Frank Sharry, the head of pro-reform America’s Voice, tells me they were “very modest” changes, arguing: “The big picture is that the more fundamental changes are being rebuffed.”

So how would you make immigration reform more conservative? Look at other changes Rubio will be pushing for in the days ahead to make reform acceptable to more Republicans. Among them, his office says, are things like mandating that specific portions of the Southwest border be fenced with double-layered fencing; tougher standards for eligibility for legal status for those with more than one misdemeanor; increased background checks; a measure to make legal status harder for those who accept state and local welfare benefits; and tweaks to eligibility standards that might end up reducing the number of people who ultimately qualify for legal status and/or citizenship.

The immigration debate is widely written about as if there aren’t really any constraints to moving the bill further to the right — as if the only people who really have to be mollified are Congressional conservatives. But Sharry of America’s Voice points out to me that moving the bill too far to the right could lose the support of labor and Democrats, potentially imperiling the bill. Sharry says that while some of Rubio’s changes might be acceptable (the double fencing, for instance, which he dismissed as a gimmick), any changes to the labor-business deal on guest workers (which some on the right are gunning for) or changes that significantly reduce the number eligible for legal status could push Dems and progressives away.

“Is there room for changes? Yes, some,” Sharry says. “But if Rubio drives the bill hard right it’s going to blow up the bipartisanship that has gotten us to this point. It’s going to fracture the progressive coalition and make it harder for Democrats to support it.”

The bottom line is that it’s not clear how much more conservative the bill can meaningfully be made at this point. The basic story remains that Republicans are going to have to accept the rough outlines of the current compromise — tons more money for border security and very stringent security benchmarks, in exchange for a very long path to citizenship — or reform isn’t going to happen.

Greg Sargent writes The Plum Line blog, a reported opinion blog with a liberal slant -- what you might call “opinionated reporting” from the left.