U.S. President Barack Obama is heading back to Washington and a contentious political fight with opposition lawmakers over his plan to unilaterally overhaul the country's immigration policies. White House aides say Obama, returning Sunday from a trip to Asia and meetings with world leaders, could act in the coming days to shield from deportation about five million migrants who are living illegally in the United States. As he left Brisbane after a G20 economic summit, Obama again said he is prepared to use his executive powers, allowing him to bypass Congress, to impose sweeping immigration reforms. He reiterated that he wishes Congress would enact comprehensive reforms, but, absent congressional action, said he would be remiss to not act on his own. "I would be derelict in my duties if I did not try to improve a system that everybody acknowledges is broken," he said. Obama said he would not be swayed from achieving new immigration rules by threats from some members of Congress to take actions that could lead to another government shutdown. Many Republican lawmakers have chided Obama, a Democrat, for his prospective immigration changes. They say that the new rules would amount to amnesty for lawbreakers. The U.S. Senate more than a year ago approved a comprehensive package of immigration reforms, but Republicans in the House of Representatives have balked at voting on the measure. Senator John McCain, one Republican who supports immigration reforms, urged Mr. Obama to "give the new Congress some time" to see if it can move forward. He warned that any executive action could have "real repercussions" for cooperation between the White House and Congress. McCain, Obama's 2008 presidential election opponent, is a senator from the border state of Arizona where many undocumented immigrants cross over from Mexico.