Former San Antonio mayor and Obama administration Secretary of Housing and Urban Development Julián Castro added himself Tuesday to a small but insistent list of Democrats calling on President Joe Biden to step aside from the race. In a series of posts to X, Castro acknowledged his gratitude for Biden’s policy accomplishments — but expressed his fear, in the wake of campaign missteps in the previous week, that he isn’t up for waging the fight he did in 2020 that swept former President Donald Trump out of office. “Joe Biden is a good man and has been a good president,” he wrote. “President Bide…