Economists often regard a broad-based price on carbon (whether in the form of a carbon tax or cap and trade) as the most efficient policy to reduce carbon dioxide emissions. Relative to a narrower policy that omits some emissions sources, a broader policy is often favored because it can exploit more low-cost emissions reduction opportunities and cause less emissions leakage to uncovered sources. Yet narrower approaches have gained considerable political support, in part because they avoid price increases for outputs (such as gasoline) regarded as especially critical to household budgets.