Donald Trump's ambitious goal to advance major tax reform through Congress during his first 100 days is already looking dubious.

Some Republicans doubt they'll have a package complete by the 200-day marker. And even the president is couching expectations by nudging his original timeline, telling Fox News' Bill O'Reilly he thinks Americans will see a tax cut by the end of the year – not within his first three or even six months.

The immense task of finding a suitable replacement for President Barack Obama's health care law, the Affordable Care Act, as well as emerging divisions between House and Senate GOP lawmakers about the exact components of a tax overhaul are the key factors forcing a potential delay.

The Trump administration's initial floating of a 20 percent tax on Mexican imports is also complicating the legislative calculus, with some key members of the Senate raising questions about how such a tariff will impact the businesses and consumers they represent.

While the House appears more inclined to include a so-called "border adjusted tax" in its version of reform, the Senate is taking a more cautious approach, with Sen. Orrin Hatch of Utah, chairman of the Senate Finance Committee, notably making no commitments and signaling he favors a deliberative process.

A border adjusted tax is one that is applied in the location of consumption rather than where the services or goods are originally produced.

"In the absence of border adjustments, exports from the United States implicitly bear the cost of the U.S. income tax while imports into the United States do not bear any U.S. income tax cost. This amounts to a self-imposed unilateral penalty on U.S. exports and a self-imposed unilateral subsidy for U.S. imports," according to a tax blueprint from Speaker Paul Ryan of Wisconsin, which was released last summer.

But Sen. John Cornyn of Texas, the No. 2 ranking Republican in the chamber, has expressed concern about the impact on the cost of crude oil , leaving him undecided on the provision.

The House Ways and Means Committee has a goal of drafting comprehensive tax legislation – including lower rates, a simplified filing code and a redesign of the IRS – by late spring or early summer, according to a Capitol Hill aide. The key will be whether they can iron out differences with the closely divided Senate on the same timeline.

"If the question is when House Republicans can introduce a bill, summer's possible. But that's way different than Congress passing a bill. They get whatever they want to out of the House because they run the show. The Senate is a very different story," says Howard Gleckman, a senior fellow at the Tax Policy Center. "The Senate Republicans are starting in a very different place."

Late in the campaign, Trump named tax relief and simplification as his top priority for passage within the first 100 days of his administration. He pledged to reduce the current seven income tax brackets to just three and greatly simplify tax forms. He also proposed slashing the corporate tax to 15 percent. But cobbling together comprehensive reform – an endeavor conservatives have been pushing for 30 years – is a much more complex task than simply implementing rate cuts.

That's why more than two dozen conservative advocacy groups sent a letter to Republican House leaders this week imploring them to "make significant progress in the first hundred days of the Trump administration toward passing comprehensive, pro-growth tax reform."

Pete Sepp, the president of the National Taxpayers Union, says he doesn't expect all the legislative questions to be resolved by April 30 – Trump's 100th day in office – but argues that it is critical for Republicans to keep showing forward motion in order to maintain momentum.

The proposed border adjustment tax gives him pause and he foresees it requiring considerable bandwidth in the overall debate.

"How would it work? Would there be exemptions? Are there types of income or transactions not subject to it? Certain questions need to be answered. It's not necessarily a cake walk. And the Republican caucus needs to hold," Sepp says.

Adam Brandon, the president of the conservative free-market group FreedomWorks and another signatory of the letter to GOP House leaders, says it remains possible for a comprehensive package to be passed before the August recess but agrees that the border adjustment tax could become the sticking point that gums up negotiations.

"For a lot of people it looks and sounds like a value added tax or a tariff. On a stand-alone, I don't think conservatives could get behind it. The trade-off I think we're going to get to is: Will you take the border adjustment tax as bad medicine to get the rest of things you've been working for for a lifetime?" he says. "The longer it lingers, the more chances there are to poke a hole in it."

Tax cuts are usually enormously popular because they are simple to understand and place money back in the pockets of American consumers. But there's a reason reform is always more precarious: because the details of any change in the tax code creates new winners and losers.

Tinkering with longtime benefits like mortgage, charitable and state and local deductions are other potential minefields that could delay or prevent such sweeping legislation from getting over the finish line. Making the plan revenue neutral will be important to the deficit hawk portion of the GOP caucus; while simply producing a lower set of rates will be prized by others.

Ryan and Rep. Kevin Brady, a Texas Republican and chairman of the House Ways and Means Committee, both continue to say they'll produce tax reform in 2017. But it appears the 100-day goal has already been vanquished.