WASHINGTON (AP) _ Republican presidential contender Bob Dole stood beside President Reagan to endorse the medium-range weapons treaty today, even as he suffered fresh defections from his campaign over his delay in backing the pact.

Dole, in comments to reporters at the White House, said he would lead the fight for ratification while Vice President George Bush ''doesn't even get a vote.''

Reagan, besieged by reporters trying to question him on the political implications of the event, said, ''I thought it was the courteous thing to do'' since he had sought the Republican Senate leader's support.

The Republican from Kansas has suffered some loss of support in the 1988 presidential campaign because of his refusal to quickly back the treaty that Reagan signed last week with Soviet leader Mikhail S. Gorbachev.

Dole said he still has some reservations about the treaty. However, when pressed by reporters, he said that if the vote were today he'd vote for the pact.

He said the Senate ''can strengthen this treaty even further without requiring renegotiations with the Soviets,'' and mentioned verification and the superiority in conventional weapons that Eastern European countries enjoy over Western Europe.

Earlier today, three Dole supporters in Rhode Island announced at a news conference in Providence that they were switching to Bush's side, and cited the treaty as part of the reason.

''The vice president, through his support of the president and the INF treaty, has clearly demonstrated his commitment to world peace,'' said Leila Mahoney, a former state GOP party chairman.

Dole said he had no idea why he had been asked to come to the White House to appear with Reagan, but said he was glad he did.

''As the Republican leader, I will lead the fight for the approval in the Senate,'' he said. ''I've been the point-man in the Senate for the president's national security program, and we've won some big fights.''

Asked about Bush and political implications, Dole said, ''I don't want to get into a debate with George Bush. We just have different roles. ... I have an active role. ... He doesn't even get a vote.''

Dole had been the only one of six Republican presidential contenders not to take a firm position on the treaty, saying he needed time to study it in detail before deciding whether to support it. Bush supports it and the other four contenders have expressed varying degrees of opposition.

Dole's reluctance to state a firm position has provoked criticism from Bush and his aides. It has led one Iowa supporters to defect in recent days to the vice president's camp and prompted another to publicly encourage Dole to hurry up and declare his backing for the pact.

The vice president's campaign staff in Iowa held a news conference Monday to criticize the senator for ''straddling'' the issue and to gloat over the defection of a Dole supporter, Ames Mayor Paul Goodland, to Bush's camp.

Goodland said he was disappointed Dole ''did not take the leadership role I expected him to take'' on the intermediate-range nuclear forces treaty.