Ted Cruz vs. Marco Rubio is on. Why this matchup is important.

If Donald Trump fades, Senators Cruz and Rubio could end up the last two GOP contenders in mid-March. They would likely steer the party in different directions on foreign policy and national security.

The long-anticipated Ted Cruz versus Marco Rubio two-man struggle appears to have finally started for real. How it ends will have major ramifications for the 2016 presidential race – and perhaps for Republican Party orthodoxy as well.

Pundits have long predicted this clash because Cruz and Rubio are natural rivals. They’re both freshman US senators from big southern states. They’re both young and articulate. They’re both Hispanic. Both want to be the last candidate standing not named Donald Trump.

Right now, they’re both vying for third in the RealClearPolitics average of major national polls, behind Mr. Trump and the slumping Ben Carson. Senator Rubio has 12.7 percent of the GOP primary vote. Senator Cruz is at 12 percent. That’s as tight as a tick on a hound dog, or something like that.

Plus, Cruz is gaining in the key first caucus state of Iowa. A recent Quinnipiac survey put him in a virtual tie with Trump for first, taking margin of error into account.

That’s something the Rubio camp can’t let go unchallenged. So they haven’t. Their weapon – er, issue – of choice has been national security.

A few days following the Paris terror attacks, Rubio said at a Wall Street Journal forum that Cruz had voted “to weaken the US intelligence programs." That’s because this spring the Texas lawmaker voted with fellow presidential aspirant Sen. Rand Paul (R) of Kentucky to end National Security Agency bulk data collection. Rubio sided with majority leader Sen. Mitch McConnell and the more traditional hawkish wing of the GOP in opposing this limitation.

American Encore, a super PAC that sides with Rubio, is now repeating this charge in an ad airing in Iowa. Cruz opted to “weaken America’s ability to identify and hunt down terrorists," the spot says.

Ouch. Asked this weekend if he had a response to this attack, Cruz said it was a shame Rubio did not support 4^th Amendment privacy rights, and then pivoted to slam the Florida senator on immigration.

Rubio’s past work on a bipartisan immigration bill that included a path to citizenship for undocumented immigrants in the US may be his biggest vulnerability, given the current state of GOP politics.

“Senator Rubio’s campaign has been desperate to change the topic from his longtime partnership with and collaboration with President Obama ... in pushing a massive amnesty bill,” Cruz said on Saturday.

On Tuesday, Cruz upped the confrontation ante, calling Rubio a proponent of aggressive “military adventurism” in the Middle East and noting that both Rubio and then-Secretary of State Hillary Clinton had pushed for dictator Muammar Qaddafi’s ouster in Libya.

“The terrorist attack that occurred in Benghazi was a direct result of that massive foreign policy blunder,” Cruz said in an interview with Sahil Kapur of Bloomberg Politics.

There you see the outlines of this political and ideological confrontation. The short-term stakes are obvious: perhaps the nomination itself. Cruz and Rubio both yearn to be the last opponent standing for outsider Trump. The party’s considerable anti-Trump sentiment would then fall to them.

Or if Trump fades (how many times have we typed that, without it happening?) Cruz and Rubio could end up the last two contenders in mid-March, or perhaps April.

Then there are the policy ramifications centered on the specifics of this conflict. These are less pronounced for immigration, since Rubio has already recanted his past support for a path to citizenship, and almost all the GOP contenders have moved to the right on the issue. For foreign policy, however, the difference between Cruz and Rubio is real.

Rubio is a hawkish interventionist – not an unusual stance for a Republican presidential aspirant. He’s pushed for a more aggressive US policy toward the Syrian civil war, including enforcement of a no-fly zone and more US arms for “moderate” rebels.

Cruz seems something a bit different. He’s perhaps less eager to engage overseas – he told Bloomberg “we have no dog in the fight of the Syrian civil war." If dictators such as Qaddafi and Hosni Mubarak of Egypt supported US interests, perhaps we should not have supported their ouster, he said.

He’s positioning himself in between Senator Paul’s libertarian nonintervention views and Rubio’s desire to spread democracy and human rights in the Middle East, according to Kapur of Bloomberg.

“Cruz’s belief is that trying to democratize those societies can be counterproductive and that US military power should be focused narrowly on protecting US interests,” he writes.

Thus Cruz and Rubio would likely steer the GOP in different directions on foreign policy and national security, despite their shared desire to kill the nuclear deal with Iran and reverse Obama’s opening to Cuba. If either wins the general election, the same would hold true for the nation.