WASHINGTON - US President Barack Obama imposed the most sweeping immigration reform in a generation on Thursday, easing the threat of deportation for about 4.7 million undocumented immigrants and setting up a clash with Republicans.In a White House speech, Obama rejected Republican critics who say his decision to bypass Congress and take executive action is tantamount to amnesty for illegal immigrants and urged them to pass comprehensive immigration reform legislation that the Republican-controlled House of Representatives has blocked."Today, our immigration system is broken, and everybody knows it," Obama said. "It's been this way for decades. And for decades we haven't done much about it."Republicans pounced quickly, charging Obama had overstepped his constitutional power a year after declaring he did not have the authority to act on his own.