ByJohn S. McClenahen Constraints on the federal budget, including the cost of Medicare reform, mean "it is not certain that President Bush will be able to get even a Republican-dominated Congress" to make permanent a variety of tax cuts set to expire during the next six years, opines Merrill Lynch & Co., New York. The cuts include a reduction in the dividend tax and a cut in the capital gains tax -- as well as the reduction in marginal tax rates and an increase in the individual alternative minimum tax exemption. The Congressional Budget Office puts the cost of extending the tax cuts at $1.564 trillion during the next decade.