And that's just the past few days.
Donald Trump still dominates the Republican race, but Rubio and Cruz are in a dead heat for second. Rubio is at 17% in the national Quinnipiac University poll released Wednesday morning. Cruz is just one point back at 16%, tied for third with Dr. Ben Carson.
As Cruz and Rubio, both Cuban-Americans and young first-term senators whose rhetorical gifts have catapulted them to the top half of the GOP field, their clearest and most immediate targets are each other.
Their feud is yet the latest example of the ongoing debate among Republicans over how best to win back the White House -- with a bold, confrontational conservative or with a bridge-builder with a record of bipartisanship.
"The fact that the presidential race could come down to Cruz and Rubio could mean a huge infusion of fresh new blood and could represent a generational change for the better," said Amanda Carpenter, a CNN analyst who worked for Cruz but is not taking sides in the race. "It's an overwhelming victory for the grassroots conservative movement."
Cruz and Rubio can both claim some tea party roots, but in the Senate they have taken different paths on their way to the presidential race. Cruz, 44, has made few friends in the Senate, spearheading a government shutdown over the Affordable Care Act in 2013 and warning his colleagues against becoming "squishes."
He called Senate Majority leader Mitch McConnell a liar and has embraced the "wacko bird," label, a moniker bestowed on him by Sen. John McCain.
"Cruz has taken a flamethrower to the Capitol, more often and more noticeably than any other insider candidate he is viewed as much more of an outsider," said Ron Bonjean, a GOP strategist. "But both Rubio and Cruz are positioning themselves for a post-Trump race. The competition is in full swing to try to out-conservative each other."
A post-Trump world, however, is far from a reality. Trump is still outpacing his rivals in state and national polls -- he leads the Quinnipiac survey with 27% -- and predictions about his demise have so far proved premature and efforts to topple him have largely backfired on his opponents.
The real estate mogul, who has established a kind of bromance with Cruz, predicted that things will look different going forward.
"Even Cruz, I think he's going to have to hit me," Trump said Monday at an event in Macon, Georgia. "It's going to be a sad day, but we will hit back -- I promise."
In Iowa, the lines of attack are on the airwaves, with dueling campaign ads over illegal immigration and national security, two core issues for primary voters. In one, paid for by the group American Encore which was founded by a Rubio supporter, the narrator criticizes Cruz for weakening national security because of his vote for the USA Freedom Act.
"I stand strongly on behalf of the ability of this government to gather intelligence on our adversaries and our enemies, especially terrorists, but also other nation states. Those keep us safer," Rubio said on Fox News Tuesday. "There are Republicans, including Senator Cruz, that have voted to weaken those programs. That is just part of the record, it is nothing personal."
In a pro-Cruz talk radio ad, Rep. Jim Bridenstine, goes after Rubio over illegal immigration, hitting him for backing amnesty and standing with Obama and other liberals on comprehensive immigration reform. Courageous Conservative PAC, which backs Cruz, paid for the spot.
Cruz has ramped up his criticism of Rubio on foreign policy, one of his perceived strengths, as he tries to carve out a position between neoconservatism and isolationism. In what could be a potent broadside, Cruz linked Rubio to Benghazi, an issue that is pure red meat to the GOP base.
"Senator Rubio emphatically supported Hillary Clinton in toppling (Muammar) Qaddafi in Libya. I think that made no sense," Cruz said in an interview with Bloomberg Politics, connecting Rubio to the Democratic frontrunner. "The terrorist attack that occurred in Benghazi was a direct result of that massive foreign policy blunder."
Polls show that Cruz has benefited most from Carson's slide --he is now second to only Trump in Iowa. For his part, Rubio is not giving any ground on any voting blocs, aiming to be a consensus candidate that can do well with evangelical voters, tea party voters as well as moderates. Rubio is also framing himself as the face of a "new American century"—he frequently invokes the story of his Cuban parents and said the country needs a new generation of leaders.
Over the last weeks, Rubio has steadily rolled out endorsements from elected officials like Reps. Darrell Issa of California and Mia Love of Utah as well as Colorado Sen. Cory Gardner -- he is second only to Jeb Bush in the endorsement race, which is often seen as the best metric for judging who has the best chance of winning a party's nomination.
Meantime, Cruz has touted the support of prominent faith leaders including Ed Lofton, a high-profile Catholic in South Carolina. And a recent email to supporters had a simple subject line -- Rush Limbaugh.
"Here is what Rush just said," the fundraising email read. "(Ted's) conservative through and through. Trustworthy, strong, confident leader, and somebody in whom you can totally depend."