Republican presidential hopeful Ted Cruz is less worried about current front-runners Donald Trump and Ben Carson and instead already focusing on Sen. Marco Rubio, R-Fla., apparently seeing him as his key obstacle to winning the party's 2016 White House nomination.

The Texas senator has increasingly upped the ante on his Florida colleague, with whom he is locked in a third-place tie, according to a national poll released on Friday by the University of Southern California and the Los Angeles Times. As a result, Cruz is increasingly targeting Rubio ahead of Tuesday's GOP debate in Milwaukee, Bloomberg reported.

"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 (Rubio) is certainly formidable in that lane; I think the Jeb (Bush) campaign seems to view Marco as his biggest threat in the moderate lane."

With his remarks, Cruz aims to position himself as a "conservative" alternative to Rubio and Bush, ostensibly the more "moderate" mainstream choices within the crowded Republican field, Bloomberg noted. In the context of a GOP primary, the latter label may well be interpreted as an insult, the business publication explained.

"Every day, more and more conservatives are uniting behind our campaign," said Cruz, who had sought Rubio's endorsement while running for the Senate in 2012. "And once it gets down to a head-to-head contest between a conservative and a moderate, I think the conservative wins."

Meanwhile, Cruz seems convinced that a showoff with Rubio will likely be the eventual scenario ahead of the Republicans' national convention, CNN noted. The Texan told CNN's Jake Tapper that many experts predicted such a matchup between the two senators, both of whom can claim a Cuban-American heritage, the news network added.

"There are a lot of political observers that are saying that and I think that's certainly a plausible outcome," Cruz said, admitting that Rubio, too, was a "formidable" candidate.