I know that none have been successful, but what is the furthest that anyone has gotten in attempting a coup and seizing power?
A single coup in the US has succeeded. In the 1890s, a race riot in Wilmington, North Carolina, some 1500 white men drove out a majority of the blacks in the city, forced the mayor to resign, and elected several white participants in the riot into the newly vacated leadership positions.
Many would argue that Bush vs. Gore was a judicial coup. Both Justices Stevens and O'Connor have have said the the criteria of the deciding the case was not a proper outcome.
http://www.huffingtonpost.com/2013/05/17/john-paul-stevens-bush-v-gore_n_3292007.html