Sen. Marco Rubio's (R-Fla.) big immigration reform push is starting to run into some conservative opposition, with a fellow GOP senator on Wednesday calling him "amazingly naive" about the issue.

"I love and respect Marco,” Sen. David Vitter (R-La.) said Wednesday on the "Laura Ingraham Show," according to the show's blog. But Vitter added: “I think he’s just amazingly naïve on this issue. This is the same formula we’ve seen before. ... Promises of enforcement never materialize, and the amnesty happens immediately.”

Vitter added that he didn't believe that the bill didn't amount to amnesty, as some pro-reform Republicans have argued.

“As soon as you give (illegal immigrants) legal status, they are here legally forever, and probably they’re citizens not that long after," Vitter said. "If Marco thinks … that’s not going happen, I think he’s nuts.”

Also on Wednesday, conservative blogger Erick Erickson came out against Rubio's efforts. Rubio has since responded to Erickson's criticisms in his own blog post.

While Vitter and Erickson remain opposed to Rubio's bill, some big-name conservative commentators who have opposed comprehensive immigration reform in the past have softened that position, including Rush Limbaugh, Mark Levin and Hugh Hewitt, and Rubio has made a point to make his case on their shows.