WASHINGTON - President Bush's statement Wednesday to Republican congressional leaders acknowledging that he should do a better job handling immigration followed a report to him by aides that the conservative base is deeply concerned about that issue.Republican House members returning from the Memorial Day recess last week told the White House that their constituents are most concerned about two issues: fiscal responsibility and immigration. In both parties, elite leadership does not appreciate rank-and-file worries about the flood of immigrants and government aid to them.