  Coronal mass ejections (CMEs) are explosive plasma phenomena prevalently
occurring on the Sun and probably on other magnetically active stars. However,
how their pre-eruptive configuration evolves toward the main explosion remains
elusive. Here, based on comprehensive observations of a long-duration precursor
in an event on 2012 March 13, we determine that the heating and slow rise of
the pre-eruptive hot magnetic flux rope (MFR) are achieved through a precursor
reconnection located above cusp-shaped high-temperature precursor loops. It is
observed that the hot MFR threads are built up continually with their middle
initially showing an "M" shape and then being separated from the cusp of
precursor loops, causing the slow rise of the entire MFR. The slow rise in
combination with thermal-dominated hard X-ray source concentrated at the top of
the precursor loops shows that the precursor reconnection is much weaker than
the flare reconnection of the main eruption. We also perform a
three-dimensional magnetohydrodynamics simulation that reproduces the early
evolution of the MFR transiting from the slow to fast rise. It is also
disclosed that it is the magnetic tension force pertinent to "M"-shaped threads
that drives the slow rise, which, however, evolves into a magnetic pressure
gradient dominated regime responsible for the rapid-acceleration eruption.
