sharepoint - ASP.NET - temporarily impersonate the app pool service ID when default it to impersonate the user? -

I have to call some code on a SharePoint site that runs under the same service ID running under the web service . By default, SharePoint impersonates the user by looking at the web page, and users do not have the necessary permissions.

What is the best way to run some code using the service id of the web application, then using the default impersonation to return?