(CNN) Former first ladies Hillary Clinton and Michelle Obama provided a signal boost on Monday to another former first lady, Laura Bush, for her opinion piece slamming the Trump administration's practice of separating families at the border.

Speaking at a women's forum award ceremony on Monday, Clinton also said Trump's claim that the Democrats were to blame for the separations was an "outright lie."

Bush wrote in The Washington Post on Sunday that the practice was "cruel" and "immoral." The scathing public admonishment from Bush was rare but drew praise from both her predecessor and successor.

"The test of any nation is how we treat the most vulnerable among us," Clinton said at an event in New York. "Laura Bush made that case eloquently in The Washington Post this weekend. ... She is absolutely right."

Obama took to Twitter to back Bush, retweeting her link to the piece.

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