“Can any explain to me why voter suppression is a partisan issue? I understand that the people who are being suppressed are people who are likely to vote as Democrats, but isn’t everyone voting the point of having a democratic political system?”
On the significance of voter suppression from Evan’s link:
As we reported last February, new voting restrictions were set to block over 1.3 million voters in four swing states—Ohio, North Carolina, Virginia, and Wisconsin— mostly by eliminating early voting periods and requiring specific forms of identification.
North Carolina’s 2016 law alone affected more than 1.2 million people: 900,000 people utilized early voting in 2012, 130,000 used same-day registration in 2008, and, surprisingly, more than 200,000 registered voters don’t have driver’s licenses. As of today, those votes are back in play.
By way of comparison, Barack Obama won the state by 15,000 votes in 2008, and Mitt Romney won by 117,000 in 2012.
“When these laws first got passed, people thought ‘what’s the big deal—I have ID.’ But the truth is that while 90 percent of us do have ID, 10 percent don’t… and it’s those folks that these laws target. Not everyone has the same advantages that we have or live in the same circles.”
In the meantime, North Carolina’s photo ID requirement, changes to early voting, same-day registration, out-of-precinct voting, and preregistration have all been put on hold. A million more voters will now go to the polls.
As I understand history and politics today, voter suppression was previously a big piece of the GOP strategy–in part where gerrymandering for wealthier districts aligned with their platform also likely meant having large spread out but very populous swaths of lower income voters (likely tending to be minorities) which Democrats would target for voter engagement.
This election’s unique in that the supporters Clinton seeks to activate overlap with the GOP’s while Sanders’ ideal voter base were the most likely to experience voter suppression. That said, you’ll still probably see a common thread:
The GOP’s aims, likely heavily shaped and influenced by the American Legislative Exchange Council (ALEC is the primary legalized loophole lobbying non-profit entity/bill mill for most major corporations) are further reinforced by the ideological interests of ALEC’s leadership (being able to reduce time for public comment and public vote, altogether discouraging the number of votes from non-wealthy people–meritocracy, Ayn Rand-esque objectivism/traditional neoconservative capitalism are held in high esteem by its founders).
Most ultraconservative/counterintuitive big legislative rushes you hear about in state legislation (rush as in: representatives introduce a giant bill just before session closes at midnight and pass it without public comment and other evaluation from other colleagues) were probably crafted by ALEC with industry lobbyists several years beforehand, and then later introduced on a state by state level. 
I suspect they feed into one another which is how voter suppression became so core to the platform.
Going back in history to before the Democratic party & GOP switched their voting base and strategies (so pre-1960 all the way back to ~1870):
As a few others brought up, unresolved legacies of racism and power control also continue to have a role in shaping voter suppression–also in the interest of maintaining political power.  While the Civil War was a military victory for the North, it was a diplomatic failure especially when considering where “states rights” took hold and the successful assassination of Abraham Lincoln and the following President’s ambivalence toward enforcing abolition & reconstruction in the south.
(Sorry both of these references are long. The documentary’s worth watching-it helped me make sense of current policy trajectories in Michigan’s capital as I work on legislative advocacy even today, and the article’s a longform that takes about 15 minutes to read if I recall correctly)
Excerpt from “Not a Tea Party, a Confederate Party”:
“After the U.S. forces won on the battlefield in 1865 and shattered the organized Confederate military, the veterans of that shattered army formed a terrorist insurgency that carried on a campaign of fire and assassination throughout the South until President Hayes agreed to withdraw the occupying U. S. troops in 1877. Before and after 1877, the insurgents used lynchings and occasional pitched battles to terrorize those portions of the electorate still loyal to the United States. In this way they took charge of the machinery of state government, and then rewrote the state constitutions to reverse the postwar changes and restore the supremacy of the class that led the Confederate states into war in the first place. 
By the time it was all over, the planter aristocrats were back in control, and the three constitutional amendments that supposedly had codified the U.S.A’s victory over the C.S.A.– the 13th, 14th, and 15th — had been effectively nullified in every Confederate state. The Civil Rights Acts had been gutted by the Supreme Court, and were all but forgotten by the time similar proposals resurfaced in the 1960s. Blacks were once again forced into hard labor for subsistence wages, denied the right to vote, and denied the equal protection of the laws. Tens of thousands of them were still physically shackled and subject to being whipped, a story historian Douglas Blackmon told in his Pulitzer-winning Slavery By Another Name.
So Lincoln and Grant may have had their mission-accomplished moment, but ultimately the Confederates won. The real Civil War — the one that stretched from 1861 to 1877 — was the first war the United States lost.”