Or at least where to look? The wikipedia page on Crassus isn't helping me much.
Election bribery was rampant in the Late Republic. Crassus almost certainly bought elections, for himself and others, and he could well afford it. He was not alone, however, as bribery was nearly a requirement for getting elected to higher office in the 70s, 60s, and 50s BCE (so argues Linderksi). Laws about bribery (Latin ambitus) shown up more and more as the 1st century BCE wears on, which in itself is a great indication of its increasing prevalence. For Lintott, electoral bribery represented a shift in the intricate politics of traditional patronage, in which a powerful figure (imagine Don Corleone from the Godfather) influences the votes of his subordinate clientes. Bribery liberated the electorate from the obligations of voting per patron, or perhaps encouraged that fission, and perhaps also empowered the common voter to effect political change. There was nothing technically wrong with, but it was morally against the practices of earlier generations of Romans (the mos maiorum).