Nathan is correct in a way. The LDS actually practiced polygamy secretly until the 1920’s (long after the first ‘manifesto’ and until the second one) and many did move to mexico, including Mitt’s family.
However, they still BELIEVE in polygamy, just only have one wife at a time. Mormon marriages are considered to be ‘for eternity’ so a man (not woman) can marry in the temple (for eternity), divorce, marry again in the temple (without first wife’s permission) for eternity and upon her death, marry again in the temple (for eternity) and have 3 wives in heaven.
Also, there is a belief that polygamy will be re-instituted in this life during the end times, which also has a Mormon POTUS, and is why some LDS I know support gay marriage because they see the next step is the legalization of polygamy.
Usually called serial monogamy when applied to other religions.
The term is "Time and eternity". There are two types of marriages, "Time" and "Time and Eternity". A marriage for time is this life only. I believe that is what you would get after a divorce.
AFAIK, the only time when there is polygamy is when a wife dies, and the widower marries again in the Temple, he would have two wives in the afterlife. (A wife who loses a husband, can only marry again for "Time" not eternity.) There was some controversy regarding that fact is how I know about it.