For Ted Cruz, the single biggest obstacle to winning the Republican presidential nomination may not be outsider candidates Donald Trump or Ben Carson, but his fellow first-term senator, Marco Rubio

After strong debate performances, Cruz and Rubio—both 44-year-old Cuban Americans—saw their poll numbers rise and interest from donors jump. They are now locked in a third-place tie, according to a national Los Angeles Times poll ahead of Tuesday's debate in Milwaukee. As a result, Cruz and his allies have begun zeroing in on Rubio. 

“As I look at the race, historically, there have been two major lanes in the Republican primary. There's been a moderate lane and a conservative lane,” Cruz told CNN on Thursday. “Marco is certainly formidable in that lane. I think the Jeb [Bush] campaign seems to view Marco as his biggest threat in the moderate lane.”

The Texas hardliner's mischievous branding of Rubio as a “moderate” is the first shot in a showdown that may unfold in the coming days of the campaign. 

The label “moderate” may seem anodyne, but in the context of modern Republican primaries, it is generally meant as an insult. Just 24 percent of Republicans self-identify as “moderate” while 70 percent self-identify as “conservative,” according to Gallup. In the era of President Barack Obama, compromising with Democrats is toxic among GOP voters, who prefer that their leaders stand up for conservative principles and have instilled fear by purging long-serving Republicans with moderate views such as Senator Bob Bennett and Senator Richard Lugar.

“Every day, more and more conservatives are uniting behind our campaign,” Cruz said. “And once it gets down to a head-to-head contest between a conservative and a moderate, I think the conservative wins.”

In a head-to-head match-up, Cruz and Rubio are statistically tied, according to YouGov—38 percent of Republicans favor Rubio and 34 percent prefer Cruz. The difference is inside the November poll's margin of error.

Rubio's Record

Given Rubio's background and voting record in the U.S. Senate, some Republicans are having trouble with Cruz's description of him as a “moderate.”

“Of course he isn't,” said Katie Packer Gage, deputy campaign manager for Mitt Romney in 2012. “Cruz is being purposely misleading. It's dishonest.”

QuickTake The Tea Party

In 2010, Rubio waged a long-shot bid fueled by right-wing groups to defeat GOP establishment favorite Charlie Crist and went on to become one of the mascots of that year's Tea Party wave. His Senate voting record is anything but moderate, featuring routine opposition to bipartisan deals to avert government shutdowns and debt default, as well as rejecting popular Democratic-led proposals such as raising the federal minimum wage and toughening equal pay for women laws.

As a result, Rubio has scored a rating of 90 percent from Heritage Action, 93 percent from the Club For Growth, and 98 percent from the American Conservative Union, three conservative groups that grade Republicans on purity. By contrast, Cruz's ratings are 98 percent, 96 percent, and 100 percent—not a huge difference. Both are far above the GOP average and rank among the most ideologically conservative senators.

“If we separate ourselves from the current universe that is the Republican Party, Rubio is at the right end of the spectrum,” said Norm Ornstein, a congressional scholar at the American Enterprise Institute.

In 2013, Rubio made a major exception and broke with conservative orthodoxy to work on a comprehensive immigration reform bill, which he later turned against in the face of intense right-wing opposition. That is shaping up to be a sticking point—a pro-Cruz super-PAC recently released a radio ad in Iowa attacking Rubio's “amnesty bill.” Cruz's campaign and various super-PACs have no shortage of money to go after Rubio, should they want to—they've raised well over $60 million combined this year.

Ornstein said Rubio's turn against the immigration bill forfeits claims he might make to being a moderate problem-solver. “It is only compared to Ben Carson, Ted Cruz, Mike Huckabee, Louie Gohmert, or Allen West that you could consider Rubio moderate,” he said. “So I guess by the standards of an insurgent outlier party, he is moderate.”

Cruz and Rubio also differ on trade. Cruz voted against fast-tracking Obama's Trans-Pacific Partnership trade deal while Rubio voted in favor of it—but neither position is disqualifying on the right.

Style Difference

The most important differences between the two men are not on policy, but rather in their respective styles. Rubio typically steers clear of combustible rhetoric and has a good reputation among Republican leaders, elites, and donors (such as billionaire Paul Singer). Cruz, on the other hand, often assails his own party's leaders as traitors to the conservative cause when they cut deals with Democrats, which has earned him the admiration of the conservative base and the scorn of GOP colleagues and leaders, including being branded a “jackass” by former House Speaker John Boehner.

Rubio rejects the idea that he's an “establishment candidate.” If true, he recently told Newsmax, “I'd be raising a lot more money.” His campaign declined to comment on Cruz's remarks about him.

Cruz sought Rubio's endorsement while running for the Senate in 2012, according to the New York Times, but won without it. The following fall, after Rubio embraced Cruz's push to force a government shutdown if Obamacare wasn't defunded, the Texan heralded the Floridian as “a critical national leader” in a floor speech on Sept. 24, 2013. Cruz said, “I don't know if there is anyone more effective, more articulate, or a more persuasive voice for conservative principles than my friend Marco Rubio.”

Before it's here, it's on the Bloomberg Terminal. LEARN MORE