The White House staff, hoping to highlight a foreign policy difference in the Democratic ticket, is drafting strategy to force a vote on Contra aid before next month's Republican National Convention.

Democratic presidential nominee Michael Dukakis and other members of his party largely have condemned President Reagan's efforts to arm the Contra rebels to put pressure on the leftist government of Nicaragua.But Sen. Lloyd Bentsen of Texas, Dukakis' choice for a running mate, has supported the administration's Contra policy consistently and seems more in step on the divisive issue with Vice President George Bush, the certain GOP presidential nominee.

The Democratic leadership in Congress led an effort in February to cut off military aid to the Contras to promote a settlement between the rebels and the Sandinista government of President Daniel Ortega. But with the peace talks in disarray, Ortega is suppressing internal critics again and the administration is moving to seek renewed aid to the guerrillas.

Thursday, White House chief of staff Kenneth Duberstein said a Contra aid bill would be one of the president's legislative priorities when he returns to Washington Sunday from a weeklong vacation in California.

But it clearly is seen as a potentially rich political issue as well, with Bentsen saying he will vote his conscience.

"We would like to have a vote in the Congress" before the Republican National Convention, starting Aug. 15 in New Orleans, Duberstein said.

Senate Republican leader Robert Dole of Kansas, a former Bush rival for the White House now mentioned as a possible running mate, has introduced a bill to provide the Contras with $27 million in food, clothing, medicine and similar supplies and another $20.1 million in military assistance.

The proposed aid would be released on Reagan's decision that the yearlong peace process, which seemed to offer potential with the signing of a truce agreement March 23, has broken down.

While not adopting the Dole proposal entirely, Duberstein endorsed it in principle, saying, "It is something that we are going to be working with the Congress to make sure that we can get as much as possible to support the peace efforts that we have under way.

"We are going to be working with Dole and with the other leaders and perhaps with some of the Democrats when they come back to the Senate next week to see what kind of package, leading with Dole and others, that we can get through the Senate and hopefully through the Congress," he said.

The Contra policy has relatively strong support in the Senate, but Democrats have organizational control and will fight to stall action. The task will be even tougher in the House, although Speaker Jim Wright, D-Texas, agreed last winter to allow a floor vote on new Contra aid if there is agreement that the peace process is dead.

Contra aid was among several major issues discussed during a White House staff meeting Thursday that Duberstein said was called to set the agenda for the presidency in the remaining months of Reagan's second term.

The president was out riding horseback in the Santa Ynez Mountains during the planning session.

"It was to look at the agenda, the items that we have ongoing, look at the legislative calendar, look at where the president should spend his (political) chips, where we recommend the president use his energies - to make sure that we put a fitting cap on the Reagan presidency six months from now," Duberstein told reporters.