Republican presidential candidate said next week's Iowa caucus could make or break his campaign, admitting for the first time that front-runner Donald Trump "could be unstoppable" if he emerges as the winner in Iowa.

Cruz made the candid assessment of the race for the Republican nomination on Monday in a meeting with evangelical pastors, Fox News reported, citing a video of the meeting obtained by the Christian Broadcasting Network and The Brody File.

Cruz, who is closely trailing Trump in Iowa polling, appealed for more support from the evangelical pastors to have a chance of beating the billionaire real estate mogul.

"If Donald wins Iowa, he right now has a substantial lead in New Hampshire, if he went on to win New Hampshire as well, there is a very good chance he could be unstoppable and be our nominee," Cruz reportedly said in the video.

"So even if you're thinking about another candidate, the simple reality is there's only one campaign that can beat Trump in this state, and if conservatives simply stand up and unite, that's everything," Cruz said.

The Texas senator described the race between himself and Trump as "an absolute dead heat."

However, three recent polls of the Iowa race show Trump increasing his lead over Cruz several weeks after Cruz managed to come very close to matching his poll numbers.

Trump now holds an 11-point lead over Cruz in the latest CNN/ORC and Fox News polls in Iowa. But pundits said Cruz could still narrow the gap if he succeeds in getting support from the remaining candidates in the race.

Young Ted Cruz sought 'world domination'

Meanwhile in another video that has gone viral on social media, a teenage Cruz is shown talking about his life goals—among them "world domination."

The video was posted online Saturday by a YouTube account called "Young Ted Cruz." It's the only video on the account, CNN reported.

Off-camera, someone asks Cruz, then a student at Second Baptist High School in 1988, what are his aspirations.

"Aspirations! Is that like sweat on my butt?" Cruz replies. "Oh, I see—what I want to do in life. My aspiration is to, oh, I don't know ... be in a teen tit film."

Then, he reconsiders. "Well, other than that? Uh, take over the world," he says. "World domination. You know: rule everything, rich, powerful, that sorta stuff."

The Cruz campaign acknowledged the video on Monday, saying the comments were all in good fun.

"Good to see that young Ted also had a great sense of humour," said Cruz campaign adviser Rick Tyler.