During the Taft administration, Democrats in the House and progressive Republicans in the Senate worked together to pass which reform that had initially been supported by the Populist Party in the 1890s?

The Sixteenth Amendment to the US Constitution.

Explanation:

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The Sixteenth Amendment to the US Constitution allows Congress to levy an income tax without apportioning it to the States according to the number of its inhabitants.

According to its idea that tariffs on consumer goods unfairly affected the humble classes, the Populist Party promoted since the end of the 19th century the establishment of a federal-level tax to replace those rates.  In this way, the tax burden shifted towards the richest classes.

On February 3th, 1913, the sixteenth amendment was ratified by the necessary votes in the Congress.

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