WASHINGTON (AP) — A simmering political crisis in Bahrain could spiral into violence, encourage meddling by Iran and weaken a key U.S. ally in the Persian Gulf, a bipartisan group of senators warned Thursday in a letter to Secretary of State John Kerry. In the last few weeks, the senators wrote, Bahrain's Sunni monarchy has "taken a series of troubling steps targeting the country's peaceful opposition, as well as nonviolent human rights defenders and members of civil society." Bahrain's government crushed the Arab Spring protests in early 2011 with the help of troops from Saudi Arabia and the United Arab Emirates. [...] the island has seen low-level unrest, protests and attacks on police.