(Reuters) - The rollout of a tax-cut plan being promoted by U.S. President Donald Trump and senior congressional Republicans was delayed on Wednesday as groups of lawmakers and special interests dug in on key provisions, insisting on changes. Here are some of the most contentious issues in the tax debate, which likely will play out over the coming weeks and months before it can garner enough support to pass Congress and be sent to Trump to sign into law. 401(k). Republicans are eyeing new limits on how much money Americans can pump each year into their 401(k) retirement accounts on a pre-tax basis. Lower limits for tax-free contributions would produce new revenue to help Republicans pay for tax cuts they have proposed for high-income earners, corporations and wealthy families’ inheritances. Taxpayers now can channel $18,000 of income tax-free into 401(k)s. Republican tax writers initially proposed lowering that cap to $2,400 and, after pushback, that number has been in flux. Firms that manage 401(k) plans, Wall Street firms that execute trades for them and the 54 million Americans who have 401(k) accounts could be hurt by such a change. Mortgages. Americans can now deduct mortgage interest from their incomes if they itemize deductions. The plan does not call for changing that directly but it does call for doubling the standard deduction, a separate tax return line that determines eligibility for itemizing. Doubling that would mean fewer Americans itemizing - and fewer deducting mortgage interest. Critics say doubling the standard deduction would make the mortgage-interest deduction a benefit for only higher-income taxpayers. The National Association of Home Builders has declared opposition to the Republican plan and proposed creating a new, non-itemized tax credit for mortgage interest. SALT. A proposal to eliminate a popular tax deduction for state and local tax (SALT) payments has threatened to derail the Republican plan. Democratic and Republican lawmakers from high-tax states, such as New York, California and New Jersey, oppose this change because it would hit their constituents hardest. Real estate interests have fought for an exemption of state and local property taxes, which is expected to make it into the bill, with a cap. Some SALT-state Republicans say they would be satisfied with an adequate property-tax deduction; others have said they will push to preserve income-tax deductibility. Pass-throughs. Republicans want to cut the tax rate for “pass-through” businesses, such as partnerships and sole proprietorships, to 25 percent from as high as 39.6 percent. Winners from this would include many pass-throughs that are small mom-and-pop businesses but it would also benefit big enterprises, such as hedge funds and real estate partnerships. Losers would be upper middle-class wage-earners unable to channel their incomes through pass-through structures. Some tax experts warn slashing the pass-through rate could unleash new tax-dodging schemes to enable Americans to do just that: run their personal incomes through pass-through structures such as partnerships, corporations and sole proprietorships. Tax negotiators are examining ways to limit what types of professional partnerships would qualify for a new pass-through rate. They also are weighing a proposal to limit the new rate to only 30 percent of business income. Business interest. Another change being evaluated by Republicans is ending or further restricting tax deductibility of business interest. This would mean business borrowers could no longer write off the interest they pay on debts. Businesses have formed a group, called the BUILD Coalition, to oppose the provision. Members include Abbott Laboratories (ABT.N), Owens-Illinois (OI.N), S&P Global (SPGI.N) and lobbying groups for private equity firms, farmers, mortgage bankers, real estate investment trusts, casinos and equipment leasing groups. Tax writers are looking at ways to exempt small businesses and allow them to continue to deduct business interest on debts. Deficit. Not long ago, most Republicans stood firmly against increasing the federal budget deficit and the national debt but analysts say their tax plan would hugely expand both. Washington was expected to collect $3.3 trillion in taxes in 2017 but spend $4 trillion, leaving a deficit of $700 billion. Previous deficits have piled up a national debt of $20 trillion. The Trump tax-cut plan would reduce federal tax revenues by $2.4 trillion in the first 10 years and by $3.4 trillion in the 10 years after that, adding greatly to the deficit and the debt, according to the Tax Policy Center, a non-partisan think tank. The plan would give a small economic boost but soon be overwhelmed by the rising federal debt burden, the center said. Helping the rich. The tax plan has struggled since it was unveiled as a rough framework in September with criticism from Democrats and social activists, who say it is a giveaway to the wealthy and corporations that hurts or neglects others. On that issue, the Tax Policy Center study said, “In 2018, all income groups would see their average taxes fall, but some taxpayers in each group would face tax increases. Those with the very highest incomes would receive the biggest tax cuts.” 