Republican warning on Trump's Mexico tariffs: Don't do it

US Republicans including powerful allies of Donald Trump pushed back strongly Tuesday against the president's threat to slap tariffs on Mexican imports, warning the move did not have the backing of Congress.Mexican and American officials were holding crunch talks in Washington on how to avoid the tariffs, which Trump said will go into effect on June 10 unless Mexico shows it is substantially reducing the flow of undocumented migrants to the southern US border.