Location: US, your choice of state if it matters (as this is a hypothetical situation, answers under any state/federal law are acceptable)
The hypothetical situation: a family member has legally obtained an absentee ballot for an upcoming presidential election, filled the form, and sealed the envelope. The relative asks you if you can send it in the mail, to which you agree. On the way to the Post Office, you then realize you know how the relative voted, don't agree with it, and decide to "lose" the ballot.
What law(s) have you broken?
P.S.: Just to be abundantly clear, this comes from a joke/meme on Facebook. No one should actually do this.