She has been called inevitable. The can’t-be-stopped candidate. Hillary Rodham Clinton is such an overwhelming favorite in presidential primary polls that most prominent Democrats are taking a pass on challenging her.

But politics can be an unpredictable business, and those of us covering the campaign have been wondering: Is there any way a Democratic challenger could beat Hillary Rodham Clinton?

We asked three of the smartest Democratic strategists we know to imagine this situation: You are running the campaign of a liberal Democrat against Mrs. Clinton. You have $50 million to spend. What is your message? How do you run against her?

The strategists agreed to sketch out a blueprint anonymously, so as not to anger Mrs. Clinton. Some common themes immediately emerged.