Is there a performance or code maintenance issue with using assert as part of the standard code instead of using it just for debugging purposes?

Is

assert x >= 0, 'x is less then zero'

and better or worse then

if x < 0:
    raise Exception, 'x is less then zero'

Also, is there anyway to set a business rule like if x < 0 raise error that is always checked with out the try, except, finally so, if at anytime throughout the code x is < 0 an error is raised, like if you set assert x < 0 at the start of a function, anywhere within the function where x becomes less then 0 an exception is raised?