American Families United Act This bill authorizes the Department of Homeland Security (DHS) or the Department of Justice (DOJ) to exercise discretion in certain immigration cases. Under this bill, DOJ or DHS may, on a case-by-case basis, exercise discretion by declining to remove a non-U.S. national ( alien under federal law) or bar an alien from entering the United States to prevent hardship for the alien's U.S. citizen spouse, parent, or child. This discretion may also be exercised if the alien is the spouse or child of a deceased U.S. citizen. For the purposes of this bill, it shall be presumed that family separation constitutes hardship. However, DOJ or DHS may not exercise this discretion if the alien is removable or inadmissible due to certain grounds, including specified crime- and security-related grounds. This exercise of discretion may be applied to an alien who was ordered removed or denied entry prior to this bill's enactment if the alien files a motion to reopen or reconsider within two years of this bill's enactment.