Weather Today: Partly sunny, cool. High 57. Winds 6-12 mph. Low 44.  
HOUSTON, Oct. 28ÛÓVice Pres-ident Bush fought off attacks from his Republican rivals tonight in the opening debate of the GOP presidential campaign, standing alone as a defender of President ReaganÛªs prospective treaty with the Soviet Union eliminating medium-range and shorter-range nuclear missiles in Europe.</br></br>Direct attacks on Bush by former Delaware governor Pierre S. (Pete) du Pont IV were met with boos. And the hall was gripped with silence when, in a sharp exchange over the arms treaty, former secretary of state Alexander M. Haig Jr. lashed out at Bush with, ÛÏI never heard a wimp out of you, George,Û when they discussed aspects of the treaty as Reagan Cabinet colleagues in 1982.</br></br>Senate Minority Leader Robert J. Dole (R-Kan.), considered BushÛªs chief rival for their partyÛªs nomination, was also a target of criticism tonight but sought to play the role of peacemaker, throwing in quips whenever the debate got tense.</br></br>Rep. Jack Kemp (R-N.Y.) and former television evangelist Marion G. (Pat) Robertson, in aggressive presentations, stressed their conservative agendas while Dole and Bush emphasized their government experience and leadership roles.</br></br>In the first face-to-face confrontation of the Republican campaign, the candidates sandwiched lively banter between serious exchanges on arms control and economic policy. The well-dressed audience in the George Brown Convention Center here rocked with laughter as Dole lured his rivals into spontaneous and self-deprecating remarks during the two-hour "Firing LineÛ broadcast carried by Public Broadcasting Systems across the nation. Host William F. Buckley Jr. and former Democratic National Committee chairman Robert S. Strauss encouraged and joined in the good-humored exchanges.