Thousands of supporters of the Shiite opposition rally in Manama, Bahrain. (Hasan Jamali — Associated Press)

Pressure is building on the Obama administration to delay a proposed arms sale to the tiny Persian Gulf nation of Bahrain, the longtime U.S. ally that cracked down on pro-democracy protesters earlier this year and that, according to rights groups, has continued to suppress dissent.

Last month, the Defense Department notified Congress of a plan to sell $53 million in armored vehicles, missiles and other military equipment to the kingdom. The sale, the notice said, “will contribute to the foreign policy and national security of the United States” and help improve security in a country that “has been, and continues to be, an important force for political stability and economic progress in the Middle East.”

The announcement drew swift condemnation from rights groups, who argued that, since crushing the blossoms of the Arab Spring in the capital Manama, the Sunni government has waged a large-scale campaign of retribution against the Shiite-led demonstrators and paid lip service to calls for reform.

This week, five Democratic senators – Robert Casey Jr. (Penn.), Richard Durbin (Ill.), Benjamin Cardin (Md.), Robert Menendez (N.J.) and Ron Wyden (Ore.) — wrote to Secretary of State Hillary Rodham Clinton to express their concern about the arms deal.

“Completing an arms sale to Bahrain under the current circumstances would weaken U.S. credibility at a critical time of democratic transition in the Middle East,” the senators wrote. “We urge you to send a strong signal that the United States does not condone the repression of peaceful demonstrators by delaying the possible arms sale until the Bahraini government releases its political prisoners, addresses the independent commission’s recommendations, and enters into meaningful dialogue with Bahraini civil society and opposition groups.”

On Thursday, Republican Sen. Marco Rubio (Fla.), a member of the Foreign Relations Committee, also weighed in, urging Clinton in a letter to delay the sale of any item that could be used to disrupt peaceful dissent until after an independent panel in Bahrain that is investigating the unrest announces its findings.

At the time of the protests, the Obama administration condemned the violent crackdown by the Bahraini government, which called in thousands of troops from Saudi Arabia and other Persian Gulf countries to put down the unrest.

In May, President Obama declared that “mass arrests and brute force are at odds with the universal rights of Bahrain’s citizens and will not make legitimate calls for reform go away.” And last month, in his speech before the U.N. General Assembly, the president said that “steps have been taken toward reform and accountability” but that “more is required.”

Actually ensuring that the kingdom lives up to its promises, however, is complicated by a host or regional considerations.

The kingdom, in addition to serving as a home base for the U.S. Navy’s 5th Fleet, is a bulwark against Iranian power in the region. And after the strain on relations with Bahrain earlier this year, a demonstration of support by the Obama administration would be welcomed by other allies in the Persian Gulf.

Jon B. Alterman, director of the Middle East program at the Center for Strategic and International Studies, said the fundamental problem regionally is that Bahrain and its allies see the uprising in the kingdom as a consequence of covert action by Iran, and efforts to put it down as necessary.

A decision by the Obama administration to delay the arms deal would be perceived as the United States turning its back on it allies. At the same time, a decision to go ahead with the deal could prompt an outcry in Washington.

“It’s hard to see how the administration comes out with a win on this, ” Alterman said. “In the near term, I see only complications.”