If Sen. Rand Paul has any hope of capturing the Republican presidential nomination in 2016, he'll have to convince the conservative base he can be trusted on foreign policy -- which is exactly why Sen. Ted Cruz is his most dangerous critic.

Though they have often been allies on domestic policy, anybody paying close attention knew that there was always a big gulf on foreign policy issues between Cruz, R-Texas, and Paul, R-Ky.

Whereas Cruz has a much more traditional Reaganite view of a strong role for America in the world, Paul seeks to advance his father's brand of non-interventionism, which advocates a more restrained U.S. role on the world stage. So it shouldn't come as any surprise that these disagreements have spilled into the open over the past week, with Cruz emphatically stating that he doesn't agree with Paul on foreign policy and Paul insisting Cruz mischaracterized his views.

It's important to keep in mind the broader historical context here. Though there has always been a subset of conservatives who have supported a more restrained, non-interventionist foreign policy, that generally hasn't been a mainstream view within the Republican Party. Over the course of two presidential campaigns, Rand's father, Ron Paul, raised his profile, but was never an actual threat to win the nomination, in no small part because his foreign policy views were out of sync with much of the party.

After winning his Senate seat in the 2010 Tea Party wave, Rand's challenge was to try to make his father's views more acceptable within the party and mount a more credible presidential campaign. To accomplish this, he's tried (with mixed success) to avoid the type of outrageous statements and controversies that doomed his father. At the same time, he's built up a following on fighting for limited government on domestic issues.

His best chance of making headway in a presidential race is to leverage the trust conservatives have for him on domestic issues to make his foreign policy views easier for conservatives to accept. If it's him debating foreign policy with the likes of New Jersey Gov. Chris Christie or other figures favored by the party's establishment, it would be much easier for Paul to muddy the waters. He could essentially argue, "Of course, big government establishment RINOs would smear my foreign policy views, because they're frightened of having a true conservative win."

That's much harder to do with Cruz in the picture. Cruz has at least as much credibility as Paul with the conservative base -- if not more. Whether or not Cruz runs, having him in the media amplifying the criticism of Paul's foreign policy views would make Paul's already difficult job of trying to appeal to a wider electorate that much harder. He cannot dismiss Cruz as just another establishment RINO trying to sabotage the candidacy of a genuine conservative. Anything Paul does to assert that he really believes in a strong role for the U.S. in global affairs risks alienating his father's energetic supporters, who favor a more restrained foreign policy. Anything he does to shore up support among this core group of his father's supporters would then feed into the criticism being lobbed by Cruz.

There's been a false impression created that Paul's non-interventionist views are gaining traction within the GOP. This idea has been based on trying to find superficial areas of agreement among Republicans (on issues such as opposing U.S. military action in Syria) that obscure fundamental disagreements. As I wrote in a column in September, a lot of conservative national security hawks opposed military intervention in Syria -- not because they shared Paul's views, but because they are more skeptical than neoconservatives of making democracy promotion a key tenet of foreign policy, and feared action would benefit Islamic militants. This is why Cruz opposed intervention at the time.

My working assumption has been that Paul isn't a serious threat to be the GOP presidential nominee in 2016, and his recent dust-up with Cruz only reaffirms that view.