WASHINGTON — Senator Obama’s endorsement of a proposal by Senator Clinton to give health care tax credits to small businesses marks the first policy-related olive branch he has offered to his vanquished rival.

The presumptive Democratic presidential nominee unveiled the plan during a speech yesterday in San Diego to the National Council of La Raza, where he touted its potential to expand health care and jobs for Latino businesses and families.

Under the $6 billion-a-year plan, small firms that provide health care for their employees would be eligible for refundable tax credits covering as much as 50% of the cost of the premiums. His advisers described it as a “complement” to his broader health care plan, which the campaign estimates would cost between $50 billion and $65 billion a year. They said the tax credits would be paid for with new and expanded measures aimed at reducing federal spending on health care.

In his speech, Mr. Obama noted that the proposal was “an idea championed by my friend Hillary Clinton, who’s been leading the way in our battle to insure every American.”

Health care provided the sharpest domestic policy disputes of the protracted Democratic primary battle between Mr. Obama and Mrs. Clinton, and the substantive and symbolic significance of his tax credit shift was underscored on multiple counts yesterday. The Illinois senator announced the proposal before a crucial constituency that Mrs. Clinton dominated in the primary — Hispanic voters — and he turned to an ardent supporter of hers, Rep. Nydia Velazquez of New York, to vouch for the plan in a conference call with reporters. Ms. Velazquez, chairwoman of the House Small Business Committee, made headlines last month when she urged Mrs. Clinton to seek the vice presidential spot on the Obama ticket, arguing that it was the only way he could secure the backing of enough Hispanic voters to win the election.

The Obama campaign did not say precisely why he did not include the tax credit proposal originally, but his director of economic policy, Jason Furman, cited the increased strain on the economy during the last year as a reason for adding it now.

“Senator Obama is very open to new ideas and open to figuring out how to make his health plan the best it could possibly be,” Mr. Furman told reporters on a conference call. “Very naturally we looked at other health plans out there, and Senator Clinton had been a champion of this issue. It was clearly a very good element of her plan, and we’re happy to take it on board.”

While yesterday’s announcement marked a step toward Mrs. Clinton on health care, Mr. Furman signaled that Mr. Obama had no intention of adopting the centerpiece of his rival’s plan — a mandate that every American purchase insurance.

“The reason that we don’t have a mandate in our plan is that we can get to universal coverage without having a mandate. Today’s proposal is one more reason why that’s the case,” Mr. Furman said, citing the added incentives for businesses and the reduced cost for individuals. “We’re absolutely convinced that we can achieve the goal that Senator Clinton and Senator Obama share, which is getting affordable, portable insurance for everyone, without needing to have a mandate.”

The proposal comes just two weeks after Mrs. Clinton’s former policy director, Neera Tanden, and other aides joined Mr. Obama’s campaign, but Mr. Furman said the tax credits had been under consideration “for some time.”

A spokesman for Mrs. Clinton, Mo Elleithee, said the New York senator was “gratified” by the Obama tax credit proposal. “Senator Clinton believes that the inclusion of such a measure makes Senator Obama’s health plan even stronger than it was before, and she salutes him for doing so,” she said.

The Obama campaign did not offer specifics about what size businesses would qualify for the tax credits or how much they would need to contribute to their employees’ premiums. It said the details of how to divide up the $6 billion it had budgeted for the plan would be worked out with the Treasury Department and Congress. An aide said that, based on similar proposals that have been offered, companies with fewer than 25 employees would be eligible for a 50% credit and the percentage would be reduced gradually for businesses with between 25 and 100 employees.

The McCain campaign has been criticizing Mr. Obama’s health care proposal for requiring businesses to pay into a national insurance pool aimed at increasing competition and reducing costs, saying it would be “devastating” to small firms. The Obama campaign has said small businesses would be exempt, but it has not detailed which ones. A McCain spokesman, Tucker Bounds, dismissed yesterday’s proposal as “an obvious and crude effort to spackle together a quick political fix,” adding: “It lacks specifics, lacks funding, and he lacks credibility.”