Despite all predictions to the contrary, Donald Trump so far has not shown any signs of dropping in the polls ahead of next year's primaries. But his GOP rivals Marco Rubio and Ted Cruz still hope that the tycoon's fall is just a question of time -- and are now jockeying to be first in line to replace Trump as their party's front-runner.

Part of both senators' strategy seems to be to go after the other, and their camps are increasingly open in their criticism -- particularly through surrogates, USA Today reported.

"I think you're going to see both Cruz and Rubio go at each other more than before," Republican strategist John Feehery predicted. "You'll see a lot of intramural sparring ... (because) most people see those two as the most likely candidates" to win the GOP nomination, Feehery told the paper.

At first glance, Cruz and Rubio actually have a lot in common, USA Today pointed out: They are both 44, serve as first-term senators, and have a Cuban-American background. But unlike Cruz, a Texas lawmaker who is frequently seen as a Tea Party favorite, Rubio, his Florida colleague, is closer to the Republican Party's establishment wing.

The Texan's insistence on his conservative credentials was evident during the Nov. 10 GOP debate in Milwaukee when he chided Rubio over the "corporate welfare" of sugar subsidies. Rubio, for his part, tries to present himself as a national security hawk and foreign policy expert and challenges Cruz on the latter's opposition to government surveillance programs.

Both candidates, meanwhile, have been careful not to offend Trump supporters, whom they might eventually need to secure the majority needed to win the GOP nomination.

Cruz, for example, on Thursday went to far as to deny having questioned if Trump had the proper "judgment" to be commander in chief even though the New York Times had posted an audio recording of comments in which he had done just that, Politico reported.

But Rubio might want to consider an alternate strategy, Feehery suggested.

"What I would try to do is make Cruz either embrace Trump or disassociate himself from Trump," he said. "Because (Cruz) is trying to play this game that he's almost like Trump but not quite, so you want to kind of make that clear to everybody where he really stands."