Sens. Kirsten Gillibrand, Elizabeth Warren and Amy Klobuchar have all been mentioned as potential Democratic presidential hopefuls -- but, in February, all three of them signed on to a letter urging Hillary Clinton to run for president in 2016.

The letter, first reported by ABC News, was signed by each of the 16 Democratic women in the Senate. Originally drafted and sent privately, it has only just come to light after one of the signatories, Sen. Kay Hagan, D-N.C., accidentally mentioned it during a speech in New York on Monday.

“All of the Senate Democratic women have written her a letter encouraging her to run,” Hagan said.

The letter is of significance not only because it showcases unanimous support for a Clinton presidential bid among the Senate's Democratic women -- three of whom might have run for president themselves in 2016 -- but because of its timing: It was sent in February, a Senate Democratic aide confirmed, just after Clinton left her post as secretary of state to begin considering her next career move.

Clinton has remained coy about her future plans, but she has lately been going through the motions of a potential presidential candidate: Reconnecting with major donors, traveling the country to deliver speeches and hone her message, and appearing on the campaign trail for her friend Terry McAuliffe, who is running for governor in Virginia.