The hackers wrote the following statement on the defaced website:
The attack seems to be coordinated as a part of #OpAntifa, a campaign that has been brought up against nationalists, racists and fascists. This is a just a few days after December 15, 2013, when Anonymous first broke into nationalist.org. The hackers claimed they had deleted all their files back then.  The following screenshot shows how the Nationalist movement website looks when it is not hacked:
At the time of posting this article, the website nationalist.org still remains defaced.