Pennsylvania's Dan Frankel, whose district covers Tree of Life synagogue attacked by Gab user, subjected to online abuse after criticizing opponent's links to social media site
Doug Mastriano has been under fire for ties to far-right social network Gab, a haven for white nationalist, antisemitic and other extremist content online
Conservative platforms popular among Trump's fiercest supporters, like Parler and Gab, known for its far-right and neo-Nazi user base, have drawn a growing number of users
Family reported concerns about self-described white nationalist's rhetoric after Pittsburgh shooting; FBI agent says he defended murder of 11 at synagogue
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