Passing csrftoken with python Requests

How do you pass a csrftoken with the python module Requests? This is what I have but it's not working, and I'm not sure which parameter to pass it into (data, headers, auth...)

import requests
from bs4 import BeautifulSoup

URL = 'https://portal.bitcasa.com/login'

client = requests.session(config={'verbose': sys.stderr})

# Retrieve the CSRF token first
soup = BeautifulSoup(client.get('https://portal.bitcasa.com/login').content)
csrftoken = soup.find('input', dict(name='csrfmiddlewaretoken'))['value']

login_data = dict(username=EMAIL, password=PASSWORD, csrfmiddlewaretoken=csrftoken)
r = client.post(URL, data=login_data, headers={"Referer": "foo"})

Same error message every time.

<h1>Forbidden <span>(403)</span></h1>
<p>CSRF verification failed. Request aborted.</p>