Democrat Ruben Gallego to challenge Kyrsten Sinema for Senate seat

Arizona Democratic Rep. Ruben Gallego on Monday plans to launch a challenge against Sen. Kyrsten Sinema, CBS News has learned. 

Gallego, an outspoken liberal Democrat, has long been critical of Sinema, who dropped her party identification as a Democrat to be an independent just after the party won the Senate last year. The Arizona senator still aligns herself with the Senate Democratic caucus, though.

Sinema said at the time that she changed her party affiliation because she "never fit neatly into any party box", but the label switch prompted an immediate backlash from many Democrats, including Gallego.