Julian Zelizer is a professor of history and public affairs at Princeton University, and author, with Kevin Kruse, of the new book "Fault Lines: A History of the United States Since 1974." Follow him on Twitter at @julianzelizer. The opinions expressed in this commentary are his own. View more opinion at CNN.

(CNN) The Congressional Budget Office has predicted the federal deficit could reach $1 trillion by the 2020 fiscal year. Once again, President Trump has exposed a myth in American politics. That myth? That Republicans are the party of fiscal responsibility.

Julian Zelizer

He is just the latest Republican president to demonstrate this. His drive for corporate tax cuts, which received strong Republican support, has blown a huge hole in the federal ledger.

As we approach the 2020 election, we will likely hear the familiar refrain from Republicans claiming their party will keep the government's books balanced while "socialist" Democrats will wreak havoc on the nation's financial stability.

But President Trump isn't the first Republican to break the mold -- he is just one of a long line of Republican presidents since Ronald Reagan who have pushed for supply side tax policies, touting the idea that lower tax rates will boost economic growth. Throw in high rates of defense spending on top of that, and the delicate balance between taxes and spending is often thrown off kilter.

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