Brownback had previously supported a short-term surge of troops if it could help achieve long-term political stability, which the Bush Administration has said it hopes a troop surge will help achieve.
But Brownback rejected that argument after meeting this week with several Iraqi leaders, including Prime Minister Nouri al-Maliki, and U.S. military commanders.
"I came away from these meetings convinced that the United States should not increase its involvement until Sunnis and Shi'a are more willing to cooperate with each other instead of shooting at each other," Brownback said.
Brownback, a conservative who is running for the 2008 Republican presidential nomination, is among a handful of Republican senators to publicly break with Bush on the president's plans to escalate U.S. military involvement.
Brownback has supported the war, but in recent weeks has moved away from the Bush Administration's positions.
He has called for dividing Iraq into three relatively autonomous zones - Kurdish, Shi'a and Sunni - within a federated country. And he also said he generally supported the findings of the Iraq Study Group last month, which Brownback said provided the U.S. the opportunity to "reset the table."