  The Moon has suffered intense impact bombardment ending at 3.9 Gyr ago, and
this bombardment probably affected all of the inner Solar System. Basin
magnetization signatures and lunar crater size-distributions indicate that the
last episode of bombardment at about 3.85 Gyr ago was less extensive than
previously thought. We explore the contribution of the primordial Mars-crosser
population to early lunar bombardment. We find that Mars-crosser population
initially decays with a 80-Myr half-life, with the long tail of survivors
clustering on temporarily non-Mars-crossing orbits between 1.8 and 2 AU. These
survivors decay with half-life of about 600 Myr and are progenitors of the
extant Hungaria asteroid group in the same region. We estimate the primordial
Mars-crosser population contained about 0.01-0.02 Earth masses. Such initial
population is consistent with no lunar basins forming after 3.8 Gya and the
amount of mass in the Hungaria group. As they survive longer and in greater
numbers than other primordial populations, Mars-crossers are the best candidate
for forming the majority of lunar craters and basins, including most of the
Nectarian system. However, this remnant population cannot produce Imbrium and
Orientale basins, which formed too late and are too large to be part of a
smooth bombardment. We propose that the Imbrian basins and craters formed in a
discrete event, consistent with the basin magnetization signatures and crater
size-distributions. This late "impactor shower" would be triggered by a
collisional disruption of a Vesta-sized body from this primordial Mars-crossing
population (Wetherill, 1975) that was still comparable to the present-day
asteroid belt a 3.9 Gya. This tidal disruption lead to a short-lived spike in
bombardment by non-chondritic impactors with a non-asteroidal size-frequency
distribution, in agreement with available evidence. [abridged]
