The letter on Thursday said the United States should be prepared to consider "tangible consequences," including reconsidering arms sales, if a recent crackdown on opposition continues.
"Bahrain's failure to address the legitimate grievances of its citizens has strained the country's social fabric and invited outside actors to take advantage of the deteriorating situation," six Democratic lawmakers and one Republican said in a letter to Kerry, a former Democratic senator, Reuters reported.
"Indeed, we believe the government's harsh crackdown on the political opposition undermines the country's stability," they wrote, calling themselves "deeply alarmed."
State Department spokesman John Kirby said he was aware of reports about the letter, but had not seen it.
Asked about whether US arms provided to Bahrain could be used against the opposition, he said, "We always have concerns about the end use of items that are inside the foreign military sales program."
Bahrain, which hosts the United States' Fifth Fleet drew US and United Nations criticism this month when it moved to strip a top Shiite cleric's citizenship and closed the main Shiite opposition group.
A State Department report, first reported by Reuters, found Bahrain's national reconciliation efforts after it crushed street protests in 2011 have stalled, and said the Western ally in the Persian Gulf has not implemented recommendations to protect freedom of expression.
“We continue to urge the government of Bahrain to reverse their recent harmful actions," Kirby said a news briefing on Thursday.
Earlier this week, the State Department said Bahrain's plan to try an activist for tweets condemning its prison system and involvement in the war in Yemen is worrisome to the United States.
The letter was led by Democratic Senator Chris Murphy and also signed by Republican Marco Rubio and Democrats Patrick Leahy, Ron Wyden, Bob Casey, Chris Coons and Tim Kaine.
It asked for more information on specific actions President Barack Obama's administration is taking to press Bahrain's leadership on the issue.