The Congressional Budget Office is a formally non-partisan unit within Congress, to help members of Congress (MOCs) figure out the likely effects their proposed legislation would have on the economy. One problem with the way things work is that the CBO must make any and all assumptions that MOCs tell them to make. Every CBO report then needs to be inspected for partisan bias.
...the CBO is going to be the subject of a major political fight over how it prepares its estimates of the economic and fiscal impact of pending legislation. As we’ll discuss below, Republicans plan to mandate that the CBO use something called dynamic scoring, which has the effect of making tax cuts look far more beneficial to the economy than they are, by effectively claiming that tax cuts boost growth, which then boosts tax receipts. It would effectively institutionalize the Laffer curve, which has been widely and repeatedly debunked.