DES MOINES, Iowa (AP) _ Aides to Vice President George Bush conceded Thursday that Pat Robertson could win the state's precinct caucuses, but predicted the former television evangelist will not win the Republican presidential nomination.

George Wittgraf, Bush's Iowa campaign manager, said the field has broken into two tiers and only Bush, Robertson and Senate Republican Leader Bob Dole have any chance of winning Iowa.

''We think there's a potential that any one of three candidates could win the Iowa caucuses,'' Wittgraf said. ''No one of the others has the potential to win the Iowa caucuses.''

Wittgraf's comments came at a news conference called by the Bush campaign to announce they've enlisted the support of two more prominent party activists. During the session, Wittgraf gave reporters a frank assessment of the status of the GOP race.

He said former Delaware Gov. Pete du Pont, New York Rep. Jack Kemp and former Secretary of State Alexander Haig have no chance of winning Iowa. Haig has conceded the state, concentrating instead on New Hampshire, but both Kemp and du Pont campaign actively in Iowa.

While Robertson has the potential to do well in Iowa, and possibly even win, Wittgraf said Bush's main mission is to defeat Dole. The Republican Party eventually will pick between Dole and Bush, Wittgraf predicted.

''It's a two-candidate race,'' Wittgraf said. ''Our first priority is beating Bob Dole in the caucuses. I find it hard to conceive of the Republican Party nominating Pat Robertson. Ultimately the decision for the Republican Party will be between George Bush and Bob Dole.''

Though some polls in recent weeks have shown Bush edging ahead of Dole in Iowa, Wittgraf said the race is still too close to call and won't be decided until just before the Feb. 8 caucuses.

''It'll be Bush and Dole and who gets hot at the end,'' Wittgraf said. He said about a third of the state's Republican activists back Bush, another third back Dole and a third are undecided.

He said those two candidates have the support of ''traditional Republicans'' who will unite to reject Robertson if one of the two front- runners stumbles.

Robertson's presence in Iowa has been an unsettling question mark for GOP activists since he easily won a straw poll of thousands of activists at a state Republican Party event Sept. 12.

Thursday's news conference was called to announce that Richard Redman, a veteran GOP activist from Carlisle who backed former Senate Majority Leader Howard Baker, and Lake Mills lawyer Richard Schwarm, a member of the Republican State Central Committee, have decided to endorse Bush.

Meanwhile, at another appearance in Kansas City, Mo., Bush made an appeal to the nation's farmers in a speech to the 60th annual Future Farmers of America convention.

''There are some in the Democratic Party who would tell you that the game is up, that American farmers can't compete with rest of the world any more,'' Bush said.

''Their solution is to pull back inside Fortress America, put up the protectionist walls of tariffs and trade barriers, and restrict your future to one of acreage reductions and subsidies and agriculture welfare.''

Bush said that trade retaliation and protectionism was an admission of defeat, and that trade barriers should be fought through negotiation.

A joint conference committee is attempting to resolve trade bills passed by the House and Senate. A controversial provision pushed by Rep. Richard Gephardt of Misssouri, a Democratic presidential candidate, would require the president to retaliate against countries that have accumulated excessive trade surpluses with the United State as a result of unfair trade practices.