(CNN) Democratic Sen. Jeff Merkley called for Homeland Security Secretary Kirstjen Nielsen to resign Monday after accusing her of telling "a lot of whoppers" at a contentious White House press briefing where she defended the Trump administration's practice of separating children from their parents at the border.

Appearing on CNN's "Erin Burnett OutFront," the Oregon senator, who had spent Father's Day touring Customs and Border Protection processing centers in the Rio Grande Valley, said that "if she's going to tell whoppers like that and keep repeating them, then she simply needs to step down. We need somebody there with integrity."

Asked by Burnett if he was outright calling for her resignation, Merkley said "absolutely, 100%."

"She's not being honest with Americans and it's so absolutely clear," he continued. "She's contradicting her President, she's contradicting her chief of staff, she's contradicting the attorney general and just making up these arguments as she goes. So yeah, absolutely,"

Merkley's demand came after he challenged multiple assertions that Nielsen made during the press briefing.

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