Daylong negotiations between representatives of the Bush campaign and conservative Republicans to forestall challenges to planks of the party platform that back President Bush's positions on stem cell research and immigration fell through last night, leading to an unexpected debate in the platform committee, two people involved in the talks said.

The debate, though brief, was a sharp contrast with the carefully choreographed displays of unity at the Democratic Party platform hearing or at the Republican subcommittees earlier in the day.

With an impassioned plea to protect what one delegate called the weakest form of human life, embryos, delegates proposed amending the platform to call for laws to block all research using embryonic stem cells.

The draft of the platform supported Mr. Bush's decision to block federal financing for research using newly obtained embryonic stem cells while allowing privately financed research or research that uses pre-existing embryonic stem cell lines.

The amendments quickly failed on a yea or nay vote after a lengthy speech by Senator Bill Frist of Tennessee, head of the platform committee and a medical doctor, about the moral complexity of the president's decision and the potential power of stem cell research.

Disappointed by the failure to work out a compromise, conservatives said they planned an even stronger push today to amend the plank that supports Mr. Bush's proposal for a ''guest worker'' program that would be open to some illegal immigrants. Conservatives critical of the plan say it amounts to an amnesty for lawbreakers, which the platform says the party opposes.

People involved in the talks said they linked compromises on both issues, seeking to exchange concessions.

Phyllis Schlafly, the longtime social conservative organizer, led efforts to revise the support for the immigration plan and to strengthen the language opposing research that uses stem cells.

Matt Schlapp, a White House political aide, was on hand in an effort to foster unity. But the talks broke off shortly before the delegates reconvened for their evening session.

Ginny Wolfe, a spokeswoman for the platform committee, said the relative unanimity of the subcommittee's meetings in the morning reflected the extent of consensus in the party. Ms. Wolfe also said an ''open door'' provision, allowing for disagreement, was a successful effort at compromise.'' Are there still differences of opinion?'' she asked. ''Yes, sure. But we are really working through them.''

Although party moderates had largely given up hope to soft the stance on issues like abortion, conservatives pressed for more, grumbling in hallways at the Javits Convention Center in Manhattan about a host of other planks, including federal spending and the military action in Iraq and Afghanistan.

In a statement, Richard Lessner, executive director of the American Conservative Union, called the platform ''a bland and uninspiring document'' that lacked ''solid conservative meat.'' Although most conservatives enthusiastically support the lengthy platform section on fighting terrorism, Mr. Lessner said, its ''open-ended commitment'' to keeping troops in Iraq and Afghanistan is troubling.

His harshest criticism fell on President Bush's plan for the new temporary worker program.

''This unfortunate initiative allows those who enter America illegally to become legal residents and apply for citizenship,'' he said. ''This idea was D.O.A. among conservatives when the president first broached it, and it is still offensive.''

In a private e-mail message that circulated among conservatives at the platform hearings and on Capitol Hill, Paul S. Teller, legislative director of the conservative House Republican Study Committee, forwarded Mr. Lessner's statement, adding:

''It confirms, as was made so clear to me during the time of President Reagan's funeral and laying in state, that President Bush has no broad vision -- and certainly no conservative vision -- for the United States of America. All he has is a random assortment of policy prescriptions, many of which contradict one another. And let's not forget his primary goal on federal spending, to cut the deficit in half in five years. Wowwee.''

Last night, Mr. Teller declined to comment on the message, except to say it was a private communication to trusted friends.

The potential effects of conservative discontent on the election remain to be seen. No conservative third-party candidate has gathered much momentum, and few conservatives are likely to pull a lever for Senator John Kerry on Election Day.

Bush campaign spokesmen often say the party is more united than ever, with support for the president comparable to their backing in Mr. Reagan's first term. Still, many Republican strategists argue that in a tight race motivation and turnout among the base voters could be decisive.

Richard A. Viguerie, the dean of its direct-mail operations, said the motivation of the Republican base was not as strong as the Democrats' this year.

''On a scale of 1 to 10,'' Mr. Viguerie said, ''the Democrats are at an 11, and I put the Republicans at about an 81/2 in terms of their enthusiasm and their motivation.''

Even though conservatives firmly dominate the party, he said, a movement could not become complacent.

''You never want to remain silent,'' he said. ''You always want to roar. After the lion has eaten his fill, he still roars to let everybody know that he is the king of the hill.''

The Republican subcommittees earlier indeed included little discord, to conservatives' frustration.

''This committee appears to be stacked,'' Ms. Schlafly said as one panel quickly passed the stem cell and immigration planks to the full committee.

Photos Chart: ''One Party, Many Views'' A look at the spectrum of Republican views on gay marriage. Republican Party Platform 2004, Draft ''. . . a few judges and local authorities are presuming to change the most fundamental institution of civilization, the union of a man and a woman in marriage. . . . anything less than a constitutional amendment, passed by Congress and ratified by the states, is vulnerable to being overturned by activist judges.'' (From the draft distributed to delegates on Tuesday) Vice President Dick Cheney ''I made clear four years ago . . . that my view was that's appropriately a matter for the states to decide . . .'' (At a campaign forum on Tuesday) President George W. Bush ''Marriage is a sacred institution between a man and a woman . . . If activist judges insist on re-defining marriage by court order, the only alternative will be the constitutional process.'' (In a White House statement on Feb. 3) Log Cabin Republicans ''The antifamily constitutional amendment targets part of the American family for discrimination. There are many reasons this amendment must be defeated.'' (From the Republican gay and lesbian rights groups Web site)