A racist photograph on the 1984 yearbook page of Gov. Ralph Northam of Virginia.

An accusation of sexual assault against Lt. Gov. Justin Fairfax.

Both reports — the first triggering an earthquake in Virginia politics last week, the second setting off an aftershock on Monday — were originally published by an obscure right-wing news site, Big League Politics, which has promoted conspiracy theories and written favorably about white nationalist candidates.

But as mainstream news outlets scrambled to confirm the photograph on Mr. Northam’s medical school yearbook page on Friday, it became clear that Big League Politics — and its mission of promoting the Trump agenda and nationalist causes — had assumed outsized influence in an increasingly Democratic state.

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The website has dealt a severe blow to Mr. Northam, who first admitted to posing for the photograph, then reversed himself and has refused to resign despite enormous pressure from fellow Democrats in Virginia and around the nation, throwing his state’s politics into a crisis.