If there is a path to new gun laws, it has to come through West Virginia and a dozen other states with Democratic senators like Mr. Manchin who are confronting galvanized constituencies that view any effort to tighten gun laws as an infringement.

As Congress considers what, if any, laws to change, Mr. Manchin has become a barometer among his colleagues, testing just how far they might be able to go without angering voters.

On Thursday a group of Democratic senators led by Dianne Feinstein of California plans to introduce a bill that would outlaw more than 100 different assault weapons, setting up what promises to be a fraught and divisive debate over gun control in Congress in the coming weeks. But a number of centrist lawmakers like Mr. Manchin have already thrown the measure’s fate into question, saying that all they are willing to support for now is a stronger background check system. …

But now, Mr. Manchin, who affirmed his support for gun rights by running a campaign commercial in 2010 showing him firing a rifle into an environmental bill, says he is not so sure. One of his local offices has been picketed, and even some of his most thoughtful supporters are cautioning him that stronger background checks are about all the gun control they can stomach.