WASHINGTON (AP) — Texas Gov. Rick Perry is calling for a flat income tax rate, tying his Republican presidential campaign to a contentious issue that excites many conservatives but mainstream America has repeatedly refused to embrace.Wealthy businessman Steve Forbes says Perry is preparing to unveil a plan designed to simplify the federal tax code and lower the rate that many people pay on income and investments. Perry plans to detail the proposal next week.Forbes is a longtime flat-tax advocate as well as a Perry campaign adviser.The main idea is to replace the stair-step range of income tax rates with one rate, paid by everyone.Liberal groups are already criticizing the idea, saying that a flat tax would raise taxes on lower- and middle-income Americans while giving breaks to the wealthiest.