Sen. Bernie Sanders (I-Vt.), a candidate for the Democratic presidential nomination, has yet to endorse Texas Democrat Jessica Cisneros , an immigration attorney challenging Rep. Henry Cuellar, one of the Houses most conservative Democrats , in Texas 28th Congressional District.

Sanders silence in what has become one of the most closely watched primary challenges in the country has puzzled and frustrated some of the progressive activists who are typically his allies.

The Vermont senators absence is particularly glaring because of the breadth of support Cisneros already enjoys. The Laredo native, who at 26 would be the youngest-ever woman elected to Congress, is backed by Sen. Elizabeth Warren (D-Mass.), Reps. Alexandria Ocasio-Cortez (D-N.Y.) and Ayanna Pressley (D-Mass.), and a broad array of progressive organizations and labor unions that typically stick with incumbents. The latter group of endorsers includes EMILYs List, the Planned Parenthood Action Fund, a branch of the Communication Workers of America, MoveOn Action, and the League of Conservation Voters.