WASHINGTON — The House and Senate tax overhaul plans are broadly similar, but crucial differences are creating headaches for Republican leaders determined to keep myriad interest groups and factions of the GOP satisfied. And then there’s the ambitious timetable they’ve set of finishing in time to get legislation to President Donald Trump by Christmas. The most politically challenging decisions involve dealing with popular and widely used tax deductions, structuring tax cuts for business and balancing personal income tax rates between middle-class families and the rich. All of these decisions come against a generous — but firm — 10-year, $1.5 trillion cap on the measure’s cost to the federal deficit.