SDLP Leader Colum Eastwood says anger over the DUP has forced people to vote for Sinn Féin in the local council elections.
He said he believes nationalist voters are "fed up" with the DUP's boycott of Stormont and wanted to send a message.
"A lot of people within nationalism are very angry, very fed up wanting to send a message to the DUP to get back to work," he told UTV.
"I think a lot people felt the best way to do that was to vote Sinn Féin and I think this was always going to happen.
"They will have a very big vote across the north."
As results continue to come in across Northern Ireland, Mr Eastwood predicted it will be a "difficult day" for the SDLP, as Sinn Féin looks to have had a strong election.
However, he said his party's vote looks to be holding up well in Foyle.
"The same story that's happening across the north is still happening in Derry but the SDLP have performed well in this city," said Colum Eastwood.
Want a quick and expert briefing on the biggest news stories? Listen to our latest podcasts to find out What You Need To Know.