WASHINGTON—President Trump’s budget blueprint for the next decade relies on up to $1.2 trillion in revenue that might not materialize.

The budget assumes the government will collect as much as that amount between 2020 and 2029 from taxes the administration opposes or from scheduled tax increases that are likely to draw strong business opposition. The $1.2 trillion estimate comes from the Committee for a Responsible Federal Budget, an independent bipartisan nonprofit.

The...