Castro, a fast-rising star within the party who gained prominence with his keynote speech at the 2012 Democratic National Convention, will headline a fundraiser hosted by Sen. Tom HarkinTom HarkinThe Hill's 12:30 Report Distance education: Tumultuous today and yesterday Grassley challenger no stranger to defying odds MORE (D-Iowa), according to the Des Moines Register.

Castro's well-received speech helped give him a national platform, and he's been a vocal supporter of immigration reform. Though Castro has ruled out a run for the presidency in 2016, his office touted a poll showing him as a popular choice to be vice president earlier this summer.