“I haven’t read his plan, I’ve only read press accounts, or some of them,” he added. “Obviously there are some ideas that have merit, but the majority of it is not a workable plan that could ever pass Congress.”

Rubio was part of a bipartisan group of senators who unsuccessfully tried to pass comprehensive immigration reform in 2013. The proposal, which passed the Senate but was never taken up in the House, included a pathway for undocumented immigrants to become citizens - a largely unpopular measure amongst Republican primary voters.

Rubio told NBC News’ Kelly O’Donnell that his past experience working on immigration reform taught him the country needs a plan for stopping people from continuing to enter the country illegally before larger legislation can be passed.

“[Americans] are not prepared to do anything about it until they know this isn’t going to continue. They don’t want to see 12 million people replaced by 12 million more 10 years from now,” Rubio told NBC News.

Trump’s call to end birthright citizenship has divided the large Republican presidential field. Bobby Jindal tweeted Monday that he also favors the proposal, while Carly Fiorina and Scott Walker have signaled support for parts of Trump’s plan as well. Jeb Bush and Lindsey Graham have spoken out against Trump’s views on immigration.

Despite a rainy morning in Des Moines, Iowa, Rubio still managed to draw almost 200 people to his soapbox speech, where he delivered an optimistic message centered on his campaign slogan of building “a new American century.”

“We can do this together, that’s why I’m asking you for your support on February 1, I’m asking you to caucus for me,” Rubio told a crowd sheltered under umbrellas.

Rubio’s visit to the opening contest of the 2016 presidential race comes as he continues to struggle to breakout of the crowded field. A CNN poll released Tuesday showed Rubio getting support from 8 percent of Republicans, fourth among GOP candidates.

Rubio told reporters he intends fully to compete in Iowa, even though his prior immigration efforts may alienate him from some of the state’s conservative voters.

“We don’t run to finish in second or third place anywhere,” he said.