Are there sources for this?

Both the Mueller report[1] and the Senate report[2]. There is apparently some question whether Wikileaks even got the RNC data and didn't release it or just didn't bother asking for it in the first place[3]. Assange himself has discussed why he was only interested in hurting the Democratic party in the US[4].

1: https://www.justice.gov/archives/sco/file/1373816/dl

2: https://www.intelligence.senate.gov/sites/default/files/docu...

3: https://www.thedailybeast.com/cheats/2016/12/10/report-russi...

4: https://theintercept.com/2018/02/14/julian-assange-wikileaks...

The intercept link is quite good in quoting his motivation, which is indeed quite contrary to the stated goal of Wikileaks being neutral.

But in [1] and [2], could you hint where in there that information is to be found? Those are quite long documents and wikileaks is not the main subject there.

It wouldn't have mattered in the RNC case even if it was leaked. It was well known that Republican party insiders disliked Trump, but Trump won anyway. The DNC leak was motivated by Bernie's treatment in the primary.

I don't see how one follows from the other. It's also rather well known that Bernie was disliked by much of the Democratic establishment, so shouldn't that, by your reasoning, mean that WikiLeaks shouldn't have published the DNC leaks either?

A shame there aren’t any other journalists that could have leaked the other party’s emails, clearly Assanges fault

I mean, yes, great example, that's the kind of sad argument Assange's defenders have to say with a straight face

This has been heavily disputed already... it's not clear that wikileaks even had access to the RNC data.