By David Lawder and Richard Cowan WASHINGTON (Reuters) - Republicans in the House of Representatives were planning to pursue on Friday a temporary funding bill to keep the lights on at the U.S. domestic security agency for three weeks, postponing the threat of a partial agency shutdown. As the clock ticked toward a midnight Friday deadline for funding the Department of Homeland Security, lawmakers said a stop-gap extension would buy time to try to work out differences between the Republican-controlled House and the Senate. Senate Democratic Leader Harry Reid, who has insisted on a full Homeland Security funding bill free of immigration reform restrictions that have caused weeks of delay, said on Thursday he would refuse to allow such negotiations with the House.