“I’m going to take probably about seven or eight more weeks and make a decision about the Senate race,” the Texas Democrat told San Antonio station KENS.
Castro has been critical of both Cruz and President Donald Trump in recent months.
He recently teased Cruz on Twitter for treating Trump with kid gloves during the Republican presidential primary.
In January, he further criticized Cruz at an event for BuzzFeed when he said “he really has not done much for the people of Texas.”
Castro’s twin brother Julian, the former Housing and Urban Development secretary in the Barack Obama White House, and Rep. Beto O’Rourke are other Democrats considering a run.
Republican Rep. Michael McCaul is considering challenging Cruz in the primary.