A major proposal to enact new tax credits for children is still an unresolved part of Republican talks on tax reform, a sign of how fluid the party's plans remain after eight months of plotting.

At issue is an initiative of Ivanka Trump, the president's daughter and a senior adviser to the president, who advocated creating a new federal benefit for child care expenses throughout the campaign and in office.

The idea wasn't part of the joint principles on tax reform that the Trump administration put out with congressional leaders before the congressional research or in the White House's recent previews of tax reform, and has never had support from House or Senate GOP leadership. But President Trump endorsed it this week in a much-hyped speech in Springfield, Missouri, and the concept also has some advocates in Congress.

In Wednesday's speech, Trump said that the tax reform will feature middle-class tax relief, and that it "includes helping parents afford child care and the cost of raising a family."

"It's one of her real big beliefs," Trump said, referring to Ivanka. "And she's very committed to that."

The Trump campaign cycled through several iterations of proposals to subsidize child care through the tax code, including complicated measures involving both tax deductions and refundable credits. A White House spokesperson declined to provide more context beyond Trump's remarks.

Nevertheless, the child care provisions would be a major part of any tax bill, not a minor detail of the kind that White House officials suggested Thursday were all that was left to hammer out in Congress. Incorporating it into a sweeping reform of the tax code would make that already daunting task significantly more difficult.

An estimate of one version of the child benefits from the Tax Foundation placed a $500 billion price tag on it, over a decade. Another calculation from the Tax Policy Center, a different outside group, pegged the cost at $115 billion.

If the child care benefit is included, those are revenues that would not be available to lowering tax rates, the main Republican goal.

Some lawmakers have met with Ivanka Trump this year to discuss a child benefit in the tax code. Republican Sen. Marco Rubio of Florida, a champion of child tax credits, commended the president for including the idea in his speech via a tweet Thursday morning.

Back in June, Rubio had touted progress in discussions revolving around child tax credits after a meeting with Ivanka Trump and other members of Congress.

In theory, there is a difference between the kinds of enlarged child tax credits that Rubio proposed during his presidential campaign and tax benefits for child care expenses. But, in practice, the Trump campaign's later versions of the child care benefit would have entailed tax savings for both working parents and families with stay-at-home-parents regardless of actual child care expenses, meaning that it would effectively be the same as a child tax credit.

Ryan Ellis, a conservative tax lobbyist who has advocated for a larger child tax credit, said that including the provision in tax reform would help make it more politically viable. The GOP, he said, isn't "doing tax reform without components to help middle class working families with kids. Basic Republican coalition politics 101."

Yet Stephen Moore, an economist for the Heritage Foundation who advised the Trump campaign on its tax reform plan, noted that including credits would cut against the goals of lowering tax rates. The overarching tax reform agenda is to eliminate deductions, credits, and other tax breaks and lower tax rates in return. Including a new family benefit would be putting one of those breaks back in.

"There's an opportunity cost," Moore said.

The larger problem, though, Republicans haven't determined whether such a large provision is included in the package despite months of talks between the White House, Senate, and House meant to settle on a joint plan.

"The fact that they're still talking about anything being in or out at this stage is troubling because we should be further along," Moore said. "The clock is ticking."