The House and Senate Wednesday overrode the majority of Nixon's veto of 29 bills and four line items, the most vetoes the governor has issued in a single session in his five years in office, The New York Times reported.
But Nixon's vetoes were successful on perhaps the two most divisive measures voted upon during the session.
Nixon argued the tax-cut bill, should it be signed into law, would reduce financing for education, mental health and other services. The bill would decrease taxes for businesses and lower the state's income tax rate for the first time in more than 90 years, the newspaper said.
"It's a defining moment," he said during a news conference. "Today was about protecting our economy, our communities and, especially, our schools from this costly and misguided bill."
The gun law would have allowed Missouri residents to own a machine gun -- illegal under federal gun laws -- and would make it illegal for federal agents to try to enforce federal gun laws, the Times said.
Nixon called the gun law unconstitutional because it violated the Constitution's supremacy clause, which gives preference to federal laws over state laws.