They're also increasingly participating in a war of words over illegal immigration and how the candidates could best address the more than 11 million undocumented workers currently living in the US.
Mr Cruz initiated the fight on Thursday, attacking Mr Rubio by name for his bipartisan work in the Senate to pass immigration reform in 2013.
He said the Florida Senator - and other members of the "Gang of Eight" leading reform - backed legalisation that would give a pathway to citizenship for undocumented workers and blocked his attempts to offer amendments to strengthen border protection.
"Talk is cheap," Mr Cruz said. "You know where someone is based on their actions."
By scheduling happenstance, Mr Rubio and Mr Cruz were the first two speakers on Friday at the Sunshine Summit presidential forum in Orlando, Florida. They offered a distinct contrast in styles.
Mr Rubio, a gifted politician, offered a polished set-piece, delivered from behind a lectern - compelling but traditional.
In contrast, Mr Cruz paced the stage with a wireless microphone. He waved his arms as he spoke and encouraged audience interaction. His appearance was more akin to a religious tent revival.
During a news conference after his speech, Mr Rubio was asked early and often about his record on immigration and Mr Cruz's criticism.
He fired back that he was "puzzled and surprised" by Mr Cruz's words, since the Texas senator was on record supporting a legal status for undocumented workers as well and that their positions aren't "dramatically different".
"Everyone running for president on the Republican side in one way or another supports legalisation of some people who are here illegally," he said.
Earlier in the day, Mr Cruz had told a radio interviewer that attempts by Mr Rubio to draw comparisons between their immigration views make him "laugh out loud".
While he supported some legal status, his objection was to Mr Rubio's support for eventual citizenship for those who had entered the country illegally.
"That's like Obama saying my position is the same as his on Obamacare," Mr Cruz said. "That's like the Ayatollah Khamenei saying my position is the same as his on the Iranian nuclear deal. It is laughingly, blazingly, on-its-face false."
At a rally in an evangelical church later on Friday, Mr Cruz announced an  immigration plan that would cap legal immigration and end automatic citizenship for those born on US soil.
He also took another not-so-veiled swipe at Mr Rubio, saying that those who supported the 2013 immigration reform effort - "including some Republicans" - were opting for the "Washington cartel" over "hardworking American men and women".
The two candidates are increasingly being viewed by pundits and Washington insiders as leading candidates to be the last two men standing when the Republican field eventually narrows. Such a theory is predicated on outsider candidates like Ben Carson and Donald Trump eventually fading, former Florida Governor Jeb Bush's campaign continuing to flounder and the other cast of candidates unable to gain traction.
If this turns out to be the case, then the fight between Mr Cruz and Mr Rubio over controlling illegal immigration - a topic that stirs passionate feelings among many Republican voters  -  isn't just a rhetorical November sideshow, it could help define how the Republican race plays out in the coming months.
Marco Rubio of Florida and Ted Cruz of Texas are both US senators, they're both the children of Cuban immigrants, they're both running for president, and they are both surging in the polls thanks to strong performances in Republican candidate debates.