DES MOINES, IOWA -- Behind the surge of Sen. Robert J. Dole's presidential candidacy so obvious at last weekend's Midwestern Republican conference is fear that Vice President George Bush is a loser.

Bush's address, showcased by the party establishment, fell flat. What's more, the dark horse who has become the most interesting Republican presidential candidate -- former Delaware governor Pierre du Pont -- offended and alarmed party regulars by suggesting out loud that the front-running vice president cannot be elected.

While Bush's agents still preach the inevitability of his nomination, based on organization and money, a different story was unfolding in the lobbies and hospitality suites. Bush faces possible defeat early next year in Michigan, Minnesota and South Dakota and a bruising battle in crucial Iowa.

The party regulars dread all this. The GOP faithful gathered here want Bush to succeed, many of them desperately. This party, conservative in style as well as ideology, feels Bush deserves hierarchical promotion.

Bob Neal, party chairman in populous Lake County, Ill., and trying to choose between Bush and Dole, was delighted by ''the best performance ever'' by the vice president. He was offended by du Pont's call for Bush to leave his ''cocoon'' -- including preferential treatment at these party conferences.

Actually, Bush delivered what seemed the longest 27-minute speech in memory. While intended as a stirring defense of Ronald Reagan and conservative principles, the prose was bureaucratic and leaden, the delivery dull and uneven.

Two years ago at the Midwestern conference in Grand Rapids, Mich., the clear alternative to Bush was Rep. Jack Kemp. But in Des Moines, all movement was to Dole (egged on by hundreds of sunflower-wearing Kansans bused up to Iowa).

Dole has polished his act since Grand Rapids, when he insulted Bush and Kemp and forecast doom and gloom. This time he was upbeat, deliberately more forceful than Bush and on his good behavior toward his fellow candidates.

But he was not demonstrably conservative, sounding like more of a moderate than Bush. Under pressure to paint a vision of the future, Dole did so -- but a vision congenial to liberal Republicans of the 1950s and similar to what today's Democrats say: ''Open opportunity for all, a world free and peaceful, an end to the nuclear nightmare, breakthroughs in health and education, science and the environment.''

If Bush's electability is questioned today, Dole's conservative credentials will be challenged tomorrow. His non-support for President Reagan in the Persian Gulf and his vote against mandatory AIDS testing -- two issues unmentioned in his speech -- foretell trouble for him on the right.

Kemp will be challenging Dole for conservatives, and his performance here was characteristically energetic. Pat Berntsen, a prominent party activist from Marion, Iowa, thought Kemp was the ''most charismatic'' of the speakers and told us she had added him to Dole and Bush on her list of possibles. But other regulars complained that Kemp was giving them, however dynamically, the same old supply-side economics they had heard so often.

Du Pont is now seriously challenging Kemp to become the non-orthodox candidate. His radical positions on welfare are more sharply honed than Kemp's and attract more attention. He showed a cool flair for battle in a televised debate here with Democrat Bruce Babbitt and what he dared say about George Bush's electability was what many others were thinking. Dole may be the party's hottest article, but he has a date in his future with a tart-tongued Pete du Pont.