The campaigns of Republican President Bush and Democratic challenger Sen. John Kerry have tentatively agreed to three face-to-face debates, a source familiar with the negotiations said early yesterday

The debates, beginning Sept. 30, could prove pivotal in the closely fought Nov. 2 election between Bush and Kerry, and in an effort to lower voters’ expectations, each side in recent weeks has already begun to portray the other as having the stronger debater.

The source familiar with the negotiations said the Bush campaign had tentatively agreed to the full complement of debates recommended by the bipartisan Commission on Presidential Debates. Bush’s campaign had originally wanted to limit the debates to two presidential ones and a vice presidential debate.

The commission said last week it needed an agreement by yesterday in order to adequately take care of the logistics of the events, but it was uncertain whether a deal would be announced yesterday. The Bush campaign denied any deal had been reached. “No agreement has been reached,” said a campaign spokesman, Reed Dickens. “When there is agreement, we will announce it.”

The commission scheduled the first 90-minute debate on Sept. 30 in Coral Gables, Fla., with a second one set for Oct. 8 in St. Louis and a third Oct. 13 in Tempe, Arizona.

A vice presidential debate between incumbent Dick Cheney and Kerry’s running mate, Sen. John Edwards of North Carolina, was set for Oct. 5 in Cleveland.

Negotiations over the debates have largely been handled by Bush’s lead representative, former Secretary of State James Baker, and Kerry’s top debate negotiator, Washington lawyer Vernon Jordan. The two sides have been discussing the format of an Oct. 8 town-hall-meeting debate, and a final agreement will depend on the outcome of these negotiations, the source said.

The Kerry campaign wanted all three presidential debates, but the Bush campaign was concerned about the Oct. 8 event in St. Louis because questioners were to include undecided voters and the Bush camp did not trust the process for choosing them.