Donald Trump is experiencing a bit of resistance from immigration hardliners, an unlikely problem at an inopportune time in the candidate's run for the Oval Office.

As the New York City real estate magnate closes in on the Republican 2016 presidential nomination, more and more conservatives are questioning his true position on the immigration issue.

Suddenly, the GOP candidate, who has vowed to erect a massive wall along the Mexican border and deport 11 million immigrants, isn't such a sure thing on immigration in the eyes of some voters.

Trump Under Scrutiny by Hardline Immigration Critics

Red flags were raised when Trump recently declared at a GOP debate that he had experienced a change of heart where high-skilled foreign workers are concerned. With that comment came a stream of news reports illustrating how Trump had used the same guest-worker programs he now criticizes on a number of his expensive projects.

Since then, Trump has advocated for a "pause" in legal immigration, but even that hasn't been enough for him to totally escape the intensified scrutiny.

"He hasn't done a very good job of connecting what he's been saying in both debates and his other press appearances and in his pep rallies ... versus what he wrote in his immigration policy," said Chris Chmielenski, director of content and activism for Numbers USA, a national grassroots group that advocates for limiting all forms of immigration.

"I think for us, what is posted on his website is very, very helpful. But the rhetoric hasn't matched," he added.

At a debate earlier this month, Trump also seemed to support a plan that would boost the flow of both high-skilled and low-skilled immigrant workers, resulting in Numbers USA lowering his grade to B+ on its scorecard.

"He doesn't know enough about the subject and won't listen to his own staffers to be able to distill a clear, coherent message," added Mark Krikorian, executive director of the Center for Immigration Studies, a think tank that calls for more restrictive immigration policies. "In other words, I'm not sure he's so much flip-flopping as just making this stuff off the top of his head."

Trump Aligns With Alabama Senator Jeff Sessions

Early in his campaign, the outspoken Trump won widespread praise from immigration critics after he adopted a policy largely crafted by hardline immigration foe Sen. Jeff Sessions, R-Ala., who has since endorsed Trump's candidacy.

Indeed, Trump's on-record proposal goes far beyond simply building a wall along the Mexican border. His policy also delves into legal immigration, requiring businesses hire U.S. citizens over immigrants for numerous guest-worker programs and boosting wages for employees with H-1B visas.

Still, Krikorian argues, "His comfort zone is the wall. Anything outside his comfort zone, he's just winging it."

Recently, Trump was also criticized for his use of the J-1 visa for foreign students program, which he reportedly used to hire labor for his upscale Terrace Restaurant in Chicago.

During the GOP debate in Detroit, rivals peppered Trump with questions about his use of another guest-worker program for lower-skilled foreign labor at his luxury club in Palm Beach, Florida.

"It's a few months, five months at the most," Trump said of the positions, in defense of his actions. "People don't want a short-term job. We want to hire as many Americans as we can, but they don't want part-time, very short part-time jobs."