Goes a big step farther sounds correct, since farther is about distance. But we're not talking distance here; it's not about about walking a step.
So further works here.
But his backing of Clinton goes a big step further than other GOP Trump skeptics in Congress.
ALSO FOUND ONLINE: Doyle sites the late Whitney Houston's widely mocked "crack is whack" interview with Diane Sawyer in 2002 as an early example.
I think the writer meant cites, not sites. At least he/she didn't use sights.
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