Republican presidential candidates are divided on Donald Trump's recently announced policy positions on illegal immigration, joining critics who question the costs and supporters who point to the value of the billionaire real estate mogul's radical proposals. 

In a six-page document published to his campaign website Sunday, Trump called, among other things, for expulsion of all immigrants living in the U.S. illegally and for Mexico to finance construction of a border wall.

The platform drew swift – but far from unanimous – reaction among the crowded field of GOP presidential contenders. Former Florida Gov. Jeb Bush characterized it as impractical, while Sen. Lindsey Graham of South Carolina called it "gibberish." New Jersey Gov. Chris Christie said he supported building a wall in some areas but that attempting to make Mexico pay for it "makes no sense." Wisconsin Gov. Scott Walker told reporters he was behind the idea of a wall and reiterated his commitment to "enforce the law." 

Few of Trump's proposals were new, mostly just setting down in position papers things Trump has said in speeches or interviews since he announced his candidacy. He called for harsher penalties for those who enter the country legally but then overstay their visas, a condition which represents the majority of immigrants in the U.S. illegally. The presidential candidate also said that until Mexico agrees to pay for construction of a wall, the U.S. should seize the money that workers in the country illegally send back to their homes and increase fees on a variety of visas and border crossing cards.

"They’ll probably just give us the money," Trump last week told Fox News' Sean Hannity of the Mexican government's reaction. "And if they don’t pay, we’ll charge ‘em a little tariff. It’ll be paid. But we need the wall.”

Mexican President Enrique Pena Nieto's office was quick to reject the idea, saying last week that the proposal "reflects an enormous ignorance" on Trump's part and that Mexico will not be paying for such a wall, which covering the 1,989 mile border would cost more than $6 billion. In addition to construction costs, it would also cost billions to maintain.

The item is one of several in the plan that critics have seized on as questionable. The Republican candidate proposed denying automatic U.S. citizenship to the children of immigrants in the country illegally, prompting many to point out that such action would require a constitutional amendment. Critics also point to other problems with his plan to deport those living in the U.S. illegally.

"I don't even know logistically how the government would go about finding 12 million people and deporting them. That's quite an expensive proposition," says Randy Capps, director of research for U.S. programs at the Migration Policy Institute.

A senior official with Immigration and Customs Enforcement estimated in 2011 that deporting an individual costs about $12,500 on average.

"So you multiply that times 12 million unauthorized immigrants and you're up in the $130 billion range," Capps says. 

Trump has pointed, in part, to crimes committed by people living in the U.S. illegally as a justification for strengthening immigration laws – a message that has resonated with some of his supporters.

Jamiel Shaw Sr., whose son was fatally shot in 2008 by a Mexican gang member in the country illegally, told CNN he supports Trump's rhetoric.

"He's speaking for the dead. He's speaking for my son," Shaw told the network. "He's speaking for the people who can't speak for themselves that demand that somebody do something."

The Heritage Foundation also noted that the underlying foundation for the Republican's immigration-reform platform was solid.

"Trump is correct that immigration law is regularly ignored," Heritage immigration expert Mike Gonzalez told NBC News. "I think a lot of people in both parties agree with that."​​​​​​​

But even the conservative think tank said the candidate's proposals were flawed in key areas.

Heritage said the U.S. economy as a whole would take a hit if immigrant workers disappeared. Immigrants make up large portions of the workforce on numerous industries, including agriculture and construction. Immigration opponents have long argued that immigrants are taking jobs that would otherwise be filled by Americans who are out of work, but numerous studies have found the contributions of immigrants to the U.S. economy to be positive rather than negative. 

A Gallup poll released last week found that 65 percent of American support a path to citizenship for immigrants in the U.S. illegally. Among them, 50 percent of Republicans support a path to citizenship for those already residing in the U.S., a statistic that sharply clashes with Trump's stance that those in the U.S. illegally must leave the country before being considered for citizenship. The Republican has said he isn't opposed to immigration, but that people have to enter the U.S. legally. Thirty percent of Republicans think all immigrants in the country should be deported, as Trump does. ​

"What Trump is doing is not helpful to grow the party because this kind of rhetoric … turns off fast-growing Latino and Asian-American populations. And it turns them off for multiple elections, not just for one election," says Karthick Ramakrishnan, a professor at the University of California-Riverside who specializes in the politics of immigration. "So it's going to be really difficult for the Republican Party to try and grow itself and be competitive for presidential politics if this is the dominant rhetoric coming out of the primary." 

Ramakrishnan says Trump is emphasizing immigration as a way to stick out in a crowded Republican field. 

"What Donald Trump is doing on immigration is not about immigration per se," Ramakrishnan says. "It's almost everything to do with securing the top spot in the Republican primaries." 

Another issue that critics raise with the premise for Trump's proposals is that immigration from Mexico is actually declining. A Pew study released in 2012 found that the figure had actually reached net zero, and possibly less. This means that any Mexican immigrants arriving in the U.S. are offset by those returning home, due to a number of factors. Those factors included the weakened U.S. job market at the time and a rise in deportations, as well as decreasing birth rates in Mexico that led fewer people to cross the border in the first place.  

Instead, many of those entering the U.S. in recent years are coming from Central America. Last summer's flood of migrants pushed immigration and border security to the fore, with almost 69,000 unaccompanied minors crossing the border in fiscal 2014. That was an increase from 39,000 in 2013 and 24,000 in 2012. Of those apprehended, 15,634 were Mexican. The most came from Honduras, at 18,244, followed by Guatemala with 17,057 and El Salvador at 16,404.  

Capps says fewer young people in Mexico are in a position to migrate, and additional U.S. resources dedicated to securing the border make it riskier to attempt the journey even if better economic opportunity lies on the other side.

Besides the immediate issues with Trump's plan, Capps points out that it does not address the future of immigration in the U.S.

"We're going to be getting a more diverse stream of immigrants from a lot of different parts of the world with a lot of different qualities and characteristics," Capps says. "So even if you did adopt his plan, it wouldn't really say much about what the future immigration flow to the U.S. should look like."