Donald Trump has accused Republican senators John McCain and Lindsey Graham of "looking to start World War III" after they criticised his travel restrictions.
The pair said on Sunday that the order halting arrivals of refugees and others from seven Muslim-majority countries may do more to help recruit terrorists than improve US security.
"Ultimately, we fear this executive order will become a self-inflicted wound in the fight against terrorism," they said in a statement.
"This executive order sends a signal, intended or not, that America does not want Muslims coming into our country. That is why we fear this executive order may do more to help terrorist recruitment than improve our security," said McCain, of Arizona, and Graham, of South Carolina.
In tweets fired back on Monday Trump said "The joint statement of former presidential candidates John McCain & Lindsey Graham is wrong - they are sadly weak on immigration.
"The two Senators should focus their energies on ISIS, illegal immigration and border security instead of always looking to start World War III."
