Why bother when it wasn't needed?
They have the motive to do it, as much or more than anyone else and certainly the capacity. They are tied to a disinformation campaigns [1] that seem to have the goal of sowing suspicion between allies who oppose them and have everything to gain by swaying our political process.
That isn't enough evidence of course, but I'm surprised by the suspicion here and I can only chock it up to the increase in cynicism that is partly a result of these very actions. Because the Bush administration managed to finagle the intelligence enough to justify the war in Iraq we can just ignore our intelligence agencies whenever we want, right?
Internal bullshit like what was going on at the DNC is annoying, frustrating and unfortunate but people are falling for the trap hook line and sinker. This happens everywhere, you can damn be sure it was going on at the RNC as well but republicans wouldn't have abandoned their party (at this point its pretty clear nothing can make a fair number of them abandon their party) but independents and some liberals are doing exactly what the hackers want (whoever they turn out to be, likely the Russians): we are getting cynical and despondent. Rather than getting involved and saying we should clean house, its like "well if you didn't suck so much you wouldn't have lost." while doing nothing to make things suck less.
[1] http://www.nytimes.com/2016/08/29/world/europe/russia-sweden...