President Bush warned Democrats Tuesday to accept his offer to allow top aides to testify about the firings of federal prosecutors only privately and not under oath, or risk a constitutional showdown from which he would not back down. "We will not go along with a partisan fishing expedition aimed at honorable public servants," Bush said in a statement from the White House. "I proposed a reasonable way to avoid an impasse." He added: "There's no indication ... that anybody did anything improper."