President-elect Donald Trump wants the Republican-controlled Congress to prioritize an extension of the tax cut law enacted during his last White House stint. With GOP majorities in the House and Senate, some sort of extension for the 2017 Tax Cuts and Jobs Act is sure to become law. However, details on what will be in — or won’t — are very much an open question.

Real policy differences exist among congressional Republicans as they gear up to craft the biggest tax legislation in eight years, and basic legislative math is tough. Republicans hold a solid but hardly overwhelming 53-47 Senate majority over Democrats. Across the Capitol, the GOP majority is effectively 220-215 once vacant seats are filled. So, there’s little room for error.