Thanks for the vague answer, it actually helped. I was previously unaware of the LiveHTTPHeaders extension, so after installing and using that I believe I found the problem. Apparently, the web request and the mechanize request were posting to two different pages.

I checked the source code of the login page and the form has an action value of seach.jsp, however the submit button actually calls a javascript function which sends the request to login.jsp.

Thanks again for your help!