Broad eligibility, ease of access to the voting process, high participation and an accurate count are all fundamental to the democratic process. Attempts to erode, diminish or compromise any of these things are nothing less than an attempt to undermine democracy. As Wendell Phillips said in 1852, eternal vigilance is the price of freedom.
The excellent podcast by Josh Briggs and Dave Roos gives a good overview of the history of voter suppression and voting rights in the US. It is a sorry tale of limited rights whose gradual and slow expansion has been intentionally impeded along the way by the intransigence of certain states and the use of various rules and procedures and constraints – literacy tests, voter registration and ID requirements and poll taxes – all intended to suppress the voting of those whose participation has been assumed to be hostile to the preservation of the existing power structure.
The reasons for voter and vote suppression are varied but have in common the urge to influence the outcome of an election by other means than political campaigning.
Voter suppression is a bipartisan activity with both sides, Republican and Democrat, claiming the other has strategic motives for any changes to voting rights or procedures. Republicans generally favour strict voter registration, voter roll purging, caging and matching techniques and ID requirements as a means of ensuring the integrity of the electoral process. Perhaps not coincidentally, such measures tend to disenfranchise and remove from the electoral process many voters who would otherwise be likely to vote Democrat. Democrats tend to favour off-cycle elections for local political bodies such as school boards because the turnout is typically lower and favours the constituents such as teachers and administrators who stand to benefit from the matters that are on the ballot (salaries, benefits). (Market Mogul)