 -- Presidential candidate Ted Cruz has praised Donald Trump for making illegal immigration a central issue of the Republican presidential race, but in an exclusive interview with ABC News, Cruz said that Trump’s immigration plan amounts to amnesty.

"So Donald Trump's position is once you deport them, it's what's called touchback. A lot of establishment Republicans had touchback," Cruz said in an interview with ABC News’ George Stephanopoulos. "Basically you make them fly back to their country for a minute, touch the ground, and then they come back with amnesty as citizens. Now, Donald is entitled to do that. He can advance that position, but he doesn't get to pretend that it's not amnesty if he's legalizing 12 million people [who are] here illegally."

Cruz also accused Trump of latching onto the issue of illegal immigration for political gain. Cruz said Trump was notably absent during the battle over the “Gang of Eight” immigration reform bill in 2013.

"I would note at that time, right in the middle of the fight, Donald Trump was publicly supporting amnesty. He was supporting Marco Rubio's Dream Act," Cruz said. "He was criticizing Mitt Romney for being too tough on immigration. That's where Donald Trump was in 2013. Now, when he launched his campaign for president, suddenly he discovered illegal immigration was a problem."

As the polls between the two men tighten, the niceties have worn off.

“Donald is getting rattled. I mean, it’s amazing. At every event, instead of actually talking about what he wants to do for the country, he just kind of stands up there and attacks me the whole time,” Cruz said.

This week, Trump called Cruz “nasty” and said “he is going down.” Cruz said he is focused on discussing his policy differences with Trump.

When asked if Trump was qualified to be commander-in-chief, Cruz said, “That's gonna be a determination that the American people are making.”

On the campaign trail, Cruz often said he is running for president to stop what he calls a “Washington Cartel." In the last few days, he’s tried to show how Trump is part of that “cartel.”

"The Washington establishment is abandoning Marco Rubio. I think they've determined that they don't think he can win," Cruz explained. "And they're rushing to Donald Trump. Bob Dole said, 'Donald Trump is someone we can make a deal with. We can cut a deal, we can work with him.'"

Cruz said he believes his campaign’s strength lies in grassroots fundraising, noting that his campaign has raised $50 million from over 700,000 contributions since last March.

“The millions of dollars of attack ads will come, but they're not gonna overcome, I believe, the strength of the grassroots,” Cruz said.