Erro 'str' object has no attribute 'exists'

Boas Caros,

Estou a ter erro abaixo. Não consigo identifica qual a causa.

  File "C:\django\connectedin\usuarios\views.py", line 20, in post
    if form.is_valid():
  File "C:\django\connectedin\usuarios\forms.py", line 20, in is_valid
    user_exists = User.objects.filter(username=self.data['nome'].exists())
AttributeError: 'str' object has no attribute 'exists'
[14/Apr/2018 18:00:08] "POST /registar/ HTTP/1.1" 500 80788

Aqui o meu forms:

from django import forms
from django.contrib.auth.models import User

class RegistarUsuarioForm(forms.Form): # Gerdar de forms.Form
    #Classe RegistarUsuarioForm

    nome = forms.CharField(required=True)
    email = forms.EmailField(required = True)
    senha = forms.CharField(required=True)
    telefone = forms.CharField(required=True)
    empresa = forms.CharField(required=True)

    #Sobrescrever método is_valid do forms.Form
    def is_valid (self):
        valid = True
        if not super(RegistarUsuarioForm, self).is_valid(): 
            self.adiciona_erro('Por fvaor, verifica os dados informados')
            valid = False

        user_exists = User.objects.filter(username=self.data['nome'].exists())

        #Verificar se o usuario existe
        if user_exists:
            self.adiciona_erro('Usuário já existe')
            valid = False

        return valid

    def adiciona_erro(self, message):
        erros = self._errors.setdefault(forms.forms.NON_FIELD_ERRORS, forms.utils.ErrorList())
        erros.append(message)