A memo from Texas Sen. Ted Cruz's chief strategist trashes Florida Sen. Marco Rubio's presidential bid.

Under the headline, "Ted Cruz is the only candidate who can defeat Trump," Jason Johnson, Cruz's chief strategist, made a case against Rubio.

"Marco Rubio cannot beat Trump," Johnson wrote. "Trump can't be beaten from the political Left, with a candidate who emphatically supports amnesty and who allows Trump to be considered the "conservative" in that context."

Johnson argues that Rubio has failed to detail a successful strategy after the first three nominating states have cast their ballots.

"Rubio is popular in the donor community and the D.C. lobbyist community, but there is no evidence that money is Rubio's problem," Johnson wrote. "They ran millions of dollars in ads attacking Cruz. Rubio was endorsed by South Carolina's very popular governor, senator, and congressman and yet, with the full support of the South Carolina establishment, he still couldn't win, losing to Trump by ten points. If Rubio can't win with all of the advantages and money in South Carolina, where can he win?"

Johnson suggested Cruz has followed through on his pledge to win over certain aspects of his base.

"In contrast to Rubio, Cruz has already delivered on his promise to energize Evangelicals and conservatives," Johnson wrote. "The results from the first three states show that only Cruz can compete with Trump for the Republican base, but can also reunite the members of the Reagan Coalition that are critical winning the nomination and the general election."

Johnson also appeared to seize on Rubio's comments that the 'children of the Reagan Revolution' are ready to lead the U.S. He touted the Texas senator's performance among 18-29-year-olds.

"The future of the party isn't just re-invigorating the Reagan coalition, it is also reaching out to younger voters," Johnson wrote. "While Rubio claims to be the agent of generational change, the data show it is actually Ted Cruz who has won the youth vote in two of the three elections thus far (Iowa and South Carolina) and he came in second with these voters in New Hampshire. Cruz has now beaten Rubio with young voters in three very different states, three times in a row."

The back-and-forth between the Cruz and Rubio campaigns heated up on Monday after Cruz spokesman Rick Tyler was sacked purportedly for propagating a story containing false information about Rubio. Tyler's subsequent apology did not save his job.

Nevadans will caucus for the GOP on Tuesday, before the Republican presidential candidates direct their full attention to the March 1 nominating contests.