Python If, elif condition for entire DataFrame column

I have a DataFrame df with columns mean and spread

I try to create a new column based on condition like

if df['mean'] > df['spread']:
    df['New Column'] = 1
elif df['mean'] == df['spread']:
    df['New Column'] = 0
elif df['mean'] < df['spread']:
    df['New Column'] = -1

It does not work, how can I process?

The error is

ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().