DURING A visit to Washington in February, Bahraini opposition leader Khalil al-Marzooq described for us how his al-Wefaqparty was seeking to bridge the growing polarization between the Persian Gulf nation’s Sunni ruling family and its restless, majority-Shiite population. In contrast to some of the groups that supported the popular uprising against the regime beginning in February 2011, al-Wefaq had firmly renounced violence and banned its members from advocating the overthrow or prosecution of the ruling al-Khalifa family. The party agreed to participate in a “national dialogue” with the government beginning in February, and Mr. Marzooq said its aim was a “power-sharing agreement” that would move the country toward constitutional monarchy.

His words offered a glimmer of hope that Bahrain, a U.S. ally that hosts the U.S. Navy’s 5th Fleet, could achieve the political compromise that has eluded other Arab states since the 2011 revolutions. Since then, however, hard-liners on both sides have driven events. Anti-government demonstrations have grown more violent, several bombings have targeted police officers and a Sunni mosque, and the regime has responded with repressive measures, including a ban on demonstrations in the capital, a lawsuit aimed at outlawing a council of Shiite clerics and restrictions on contacts between opposition groups and foreign governments and organizations.

Now Mr. Marzooq has been arrested. On Tuesday he was charged with “inciting and advocating terrorism.” Predictably, al-Wefaq and other opposition groups withdrew from the national dialogue, which in seven months had yet to reach consensus even on an agenda. The charges against the former parliamentarian are perverse: They appear to be based on a speech he delivered this month in which he was said to have waved the flag of a militant youth group. Never mind that Mr. Marzooq told the militants in the speech that “the difference between us is violence” and called on them to “persist with your nonviolence.”

Bahrain’s leaders regularly assure the Obama administration that they are open to reforms and compromise with their opposition. But massive human rights violations, including the torture of detainees, continue. Leading political figures and human rights advocates remain imprisoned. The arrest of Mr. Marzooq reinforces the growing evidence that the regime is intent on eliminating the very moderates with whom it should be bargaining.

Despite occasional criticisms and a temporary suspension of some military sales, the United States has tolerated this behavior. Fearful of compromising a naval base that backs up U.S. operations in the Persian Gulf, the Obama administration refuses to hold the Khalifa regime accountable. Incredibly, the State Department’s response to Mr. Marzooq’sarrest was to express disappointment with the opposition for withdrawing from the national dialogue. That myopic attitude will only encourage more repression and further political polarization in Bahrain — and endanger the very U.S. assets that the administration sees itself as protecting.