Legally able to work in US?

Think this is an easy question... is there any problem with asking an applicant if they are legally able to work in the US ? I know we can't ask for documentation until they are hired, but can we ask before we make them an offer?
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Then after they accept a position, we remind them for their first day to "bring documentation of proof that you have the right to work in this country."
Short and sweet but effective.