Democrats who hope Sen. Lloyd Bentsen will help the ticket win his home state of Texas and perhaps the presidential election could do well to regard the fate of Henry Gassaway Davis. In case anyone doesn't remember, Davis was Democrat Alton B. Parker's running mate when Parker ran for president against Republican Teddy Roosevelt in 1904. The voters chose between Roosevelt and Parker, and their choice was Roosevelt by a 3-2 margin. Experts concede that the vice presidential debate in Omaha, Neb., on Wednesday between Bentsen and Republican Dan Quayle may make a difference, but they say the voters' final decision will come down to a choice between the top of the tickets, Democrat Michael Dukakis and Vice President George Bush. ``Unless Quayle is absolutely destroyed in the debate with Bentsen, I don't think it will hurt Bush,'' said Leo Ribuffo, a specialist in political history at George Washington University in Washington. ``People will be looking for Quayle to blow it,'' said Samuel H. Kernell, professor of political science at the University of California at San Diego. ``If he can get through the debate in a competent fashion where he looks unexceptional, it will be a real success for him.'' Eddie Mahe Jr., a Republican political consultant, said, ``I think it is reasonable to suggest that George Bush would have won Indiana (Quayle's home state) without Mr. Quayle, and with Mr. Bentsen, Mike Dukakis is still not going to carry Texas. So you wash that out.'' ``I think the odds are that Quayle will have little or no effect on the ultimate choice,'' said Thomas Mann, program director for governmental studies at the Brookings Institution here. Elspeth Rostow, a political scholar at the Lyndon B. Johnson School of Public Affairs at the University of Texas in Austin, said Bush's choice of Quayle ``doesn't seem to have too much effect on the voters, at least so far.'' ``I don't think, if indeed Bush was disposed to choose Quayle to appeal to women, that so far it has had that effect,'' Ms. Rostow said. ``I think the conventional wisdom that the selection of a running mate is rarely decisive one way or another is absolutely true,'' said Ribuffo. A survey conducted this year by the Hearst Corp. supports this view. Eighteen percent of the 1,001 voters surveyed said they had changed their minds about a presidential candidate at one time or another because of their opinion of his running mate. Of those, 70 percent said they voted against that ticket. Political experts agree that if a running mate has any effect, he is more likely to hurt than help. Examples cited include President Ford's selection of Sen. Robert Dole of Kansas, who was criticized for his slashing debating style in 1976, and Walter Mondale's choice of Geraldine Ferraro, who suffered from examination of her family's finances in 1984. ``Clearly in my opinion Bob Dole in 1976 in that debate (with Mondale, Jimmy Carter's running mate) cost Jerry Ford votes,'' Mahe said. ``Clearly in 1984 in my opinion, Geraldine Ferraro cost Walter Mondale votes probably.'' In Richard Nixon's unsuccessful campaign of 1960, some thought that Republican vice presidential candidate Henry Cabot Lodge hurt the ticket's chances by pledging that Nixon would appoint a black to the Cabinet _ a statement Nixon disavowed. Lodge, a former ambassador to the United Nations, also was criticized for taking naps and not campaigning hard enough. ``Some have thought that if he had used his prestige from the U.N. more effectively, Nixon might have edged out Kennedy,'' Ms. Rostow said. There are exceptions to the rule that running mates don't help much, the experts said. While there is disagreement over whether Lodge hurt Nixon in 1960, almost everybody agrees that Lyndon B. Johnson helped Kennedy by enabling him to carry Texas. ``I don't think Kennedy would have won without Johnson,'' Ribuffo said. Also, Ribuffo said, Mondale may have helped Carter win election in 1976 by imparting legitimacy to the former Georgia governor in the eyes of skeptical liberals. Then there was the running mate that Republicans chose in 1900 to help the colorless William McKinley win a second term against the dynamic Democrat, William Jennings Bryan. The ticket of McKinley and Roosevelt became known as ``the kangaroo ticket because it had a kick in the tail,'' Ms. Rostow said. There haven't been many tickets in any other year, including 1988, with that much of a kick in the tail, the experts agree.