Conservatives said the Russia Tax Cut "would pay for itself".

How many times must they lie before they are excluded from policymaking?

Why 2018 Was an ‘Unusual Year’ for US Tax Revenues
Federal tax revenues were lower in 2018 than in 2017, reversing a trend of rising revenues seen in recent years, according to U.S. Treasury data tracked by District Economics Group.
http://www.thefiscaltimes.com/2019/0...S-Tax-Revenues

So just like in 2001 when Bush cut taxes, revenue dropped below the previous year's level, contradicting Conservatives claims that the cost of the tax cuts would be made up with an increase in revenue.

Conservatives lie because they are simply just stupid, bad people.