Two Republican presidential contenders, Rep. Jack Kemp (R-N.Y.) and former Delaware governor Pierre S. du Pont IV, swiftly taking issue with Vice President Bush, said yesterday that they are not satisfied with the arms control treaty the Reagan administration is negotiating with the Soviet Union.

"George has been asking people to support a treaty that hasn't been written yet. He is taking a 'Sign now, negotiate later' approach to dealing with the Soviets," du Pont said in a statement issued by his campaign.

"I accept George's challenge to support the administration's proposed INF {intermediate nuclear forces} treaty, but I will not be able to do so unless there is a link to reducing the advantage the Soviet Union has in conventional forces," du Pont said in a Iowa campaign stop.

Kemp, campaigning in Cedar Rapids, Iowa, repeated longstanding reservations about the treaty and said he wanted to debate Bush on "arms control beyond the INF agreement to include strategic and conventional weapons, Soviet behavior, the future of NATO" and President Reagan's proposed Strategic Defense Initiative (SDI), the space-based missile defense system.

Both men were responding to Bush's challenge on Tuesday for all contenders for the 1988 Republican presidential nomination to say where they stand on the prospective agreement to eliminate medium- and shorter-range missiles.

"I want to press my opponents in this race to say whether they are for it or against it, to challenge them to say whether they are going to stand for verifiable arms reduction or whether they are going to be off trying to find some excuse not to be for it," Bush said Tuesday while campaigning in Iowa.

Bush said Reagan was "not going to do a dumb deal."

Bush's challenge to his rivals, issued on the second day of his formal campaign, coupled with the swift response by Kemp and du Pont, ensures that the treaty will be a campaign issue in the Republican field.

Bush's decision to take the offensive on the issue apparently reflects his view that conservatives are making a political mistake by coming out in opposition to an arms control treaty negotiated by Reagan, a staunch anticommunist.