By Patricia Zengerle WASHINGTON (Reuters) – U.S. Republicans will have the power to block aid to Ukraine if they win back control of Congress, but they are more likely to slow or pare back the flow of defense and economic assistance than stop it, analysts said. They might also use support for the Ukrainian war effort as leverage to force Democrats to back Republican priorities such as clamping down on immigration across the southern border with Mexico. Democrats have been raising the possibility Republicans would curb assistance to Kyiv for months, given polls showing the party likely to end t…