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(AP) – As Heidi Heitkamp eked out a narrow win in last year’s North Dakota Senate race, there was one topic that rarely came up: gun rights. But now that Heitkamp is a freshman U.S. senator in Washington, the issue has become a defining one.

Heitkamp joined three fellow Democrats and a majority of Republicans this week in opposing legislation in the Senate that would have expanded federal background checks for gun purchases. Her opposition stood out because Heitkamp was the only Democrat not up for re-election in 2014 to vote against the measure. And in doing so, she shook off persistent pressure from her party’s liberal base, fellow Democratic senators and the White House.

Heitkamp says her vote was about standing firm for the Second Amendment rights of North Dakota residents.