When Bill Clinton was president, we argued about the meaning of "is." Since George W. Bush's presidency, we've been arguing about the meaning of "amnesty." Presidential candidates Ted Cruz and Marco Rubio are just the two latest Republicans to get caught up in the confusion.

Virtually all Republicans today say they oppose amnesty for illegal immigrants, even though Bush, 2008 Republican nominee John McCain, and many GOP senators supported legislation that was widely described as amnesty. Sometimes for good measure, today's Republicans say they're against "blanket amnesty."

Everything here hinges on what the meaning of amnesty is. Let's explore it.

Opponents (and I'm one of them) of "comprehensive" immigration bills — like Rubio and Chuck Schumer's Gang of Eight legislation, or Bush-era golden oldies like McCain-Kennedy — define amnesty as adjusting the legal status of undocumented immigrants so that they may stay in the country and work indefinitely.

Supporters of these immigration proposals counter that this is only amnesty if it comes without any conditions or penalties. Some say it isn't amnesty without a special path to citizenship that differs from that followed by legal immigrants.

There are two issues with this stricter definition of amnesty. First, most people accepted that the 1986 immigration legislation signed by Ronald Reagan was an amnesty and it was not totally unconditional. That leads to the second problem: Legalization of most illegal immigrants is precisely what is being debated; nobody is actually proposing or seriously discussing amnesty in the way that the comprehensive reformers define it.

It's true that the immigration proposals of the past 10 years have generally required prospective beneficiaries to jump through more hoops to adjust their legal status, though there have been questions about how rigorously these conditions were ever going to be enforced. But we're talking a difference of degree rather than kind.

All this has come up again because Rubio and Cruz are vying to be the main alternative to Donald Trump in the Republican presidential race. But there can only be one. Rubio was part of the Gang of Eight, the last bipartisan legislative initiative that was described by opponents as amnesty. Cruz opposed that initiative then, and realizes immigration is a political liability for Rubio now.

But Cruz has a problem, too: He introduced an amendment to the Gang of Eight that would have stripped the path to citizenship but still legalized illegal immigrants. Here's how Cruz described it to The New York Times in 2013: "The amendment that I introduced removed the path to citizenship, but it did not change the underlying work permit from the Gang of Eight." Cruz also clarified that "those 11 million [illegal immigrants] under this current bill would still be eligible for [registered provisional immigrant] status" and also eligible for legal "status and indeed under the terms of the bill they would be eligible for [lawful permanent resident] status as well, so that they are out of the shadows."

That's not amnesty according to the definition favored by the kind of Republican who favored the Gang of Eight bill and its predecessors. But it's certainly amnesty according to the bill's strongest conservative opponents.

Cruz and his supporters have since described the amendment as a poison pill, and the Texas senator admitted to The New York Times it could be so characterized at the time: "If your objective is actually to pass a bill insisting on a path to citizenship, it is in both intent and effect a poison pill."

Rubio backers, for their part, insist this shows there was little daylight between Cruz and their man on Gang of Eight and legalization.

The truth is somewhere in the middle. Cruz always opposed Rubio's bill as written and played an important role in its defeat. He clearly understood the importance of the path to citizenship to its Democratic supporters and knew they would never accept his compromise offer of green cards with no right to naturalization. He hoped to expose supporters' commitment to citizenship. But his contemporaneous statements make plain that this middle ground was his actual substantive policy preference and not solely a ploy to sink the Gang of Eight.

Cruz has always been circumspect in his discussions of legalization. Even during Tuesday night's debate, he eschewed an unequivocal "read my lips" pledge and instead offered the more lawyerly "I have never supported legalization, and I do not intend to support legalization." Perhaps he doesn't want to later contend with George H.W. Bush's "read my lips" problem?

As my colleague David Drucker has noted, while Cruz fights fierce battles with the left he prefers to carefully navigate issues that divide conservatives. Immigration is one such issue. Just as Cruz once favored increasing legal immigration levels and now would freeze current levels in place until the U.S. labor market tightens, he remains anti-amnesty but now adheres to a stricter definition of the term.

In other words, as the conservative mood on immigration has hardened so has Cruz's position. He should just acknowledge his evolution. Rubio went from an enforcement-first position during his 2010 Florida Senate race to the Gang of Eight to saying he learned his lesson from the Gang's failure.

But most conservatives had already learned from the mistakes of 1986. If amnesty wasn't accompanied by greater enforcement, reduced illegal immigration, and political benefits for Republicans under Reagan, how likely was any of that to occur under Barack Obama? Cruz's rightward shift on the issue has been more subtle.

Rubio's immigration dilemma is similar to Mitt Romney's on RomneyCare in 2012 — he can't celebrate his biggest legislative initiative. Cruz should avoid a Mitt Romney dilemma of his own. Conservatives were never unhappy that Romney moved in their direction on issues. They simply did not want him to insult their intelligence by pretending his past positions never existed.

Ted Cruz, take note.