On the eve of nationwide hearings that could determine the fate of his immigration bill, President Bush is signaling a new willingness to negotiate with House Republicans in an effort to revise stalled legislation before Election Day. Republicans both inside and outside the White House say Mr. Bush, who has long insisted on comprehensive reform, is now open to a so-called enforcement-first approach that would put new border security programs in place before creating a guest worker program or path to citizenship for people living in the United States illegally. "He thinks that this notion that you can have triggers...