It is no more difficult for a Mexican to come work in the US than people from any other country. In fact, thanks to NAFTA, there is even a special category for Canadians and Mexicans to come here and work in certain positions. Yes, it may take longer to get a Green Card (though most Chinese and Indian workers have an even longer wait), and a family based green card can take quite a while, but bonafide skilled workers don't have issues.
What they meant to say is that it is difficult for non-skilled, non-agricultural workers to come to the US legally, and honestly, it should be. We have plenty of people in the US who can do "unskilled" work, such as food processing, cleaning, landscaping, etc. If there was really a need there, we could create a visa category, but as it stands, there is no such need. My mom cleaned hotel rooms when she was younger, and so could plenty other Americans. Yes, they'll demand a reasonable wage, but I would think that the liberals would support that.