Promote the general welfare is the clause in the Preamble to US Constitution, not the Declaration of Independence.

The Taxing and Spending Clause grants Congress the power to levy taxes to pay debts and provide defense and welfare. This clause, found in Article 1, Section 8, Clause 1 of the U.S. Constitution, is commonly referred to as the General Welfare Clause.

the elastic clause

It is the Necessary and Proper Clause or General Welfare Clause which is also called implied powers of Congress.

The Taxing and Spending Clause is the clause that gives the federal government of the United States is power of taxation. The component parts are known as the General Welfare Clause and the Uniformity Clause.