Senate Majority Leader Mitch McConnell (R., Ky.) called the $21 trillion national debt and ballooning budget deficit "very disturbing" in an interview Tuesday, blaming "bipartisan reluctance" to embrace entitlement reform.

The U.S. budget deficit expanded to $779 billion in fiscal year 2018, the Treasury Department announced Monday, a $113 billion jump from the previous year and the worst number since 2012. CNBC reported the budget shortfall amounted to 3.9 percent of U.S. gross domestic product.

Bloomberg's Kevin Cirilli read out the numbers and asked McConnell simply, "What's going on with the debt?"

"It's very disturbing, and it's driven by the three big entitlement programs that are very popular: Medicare, Social Security, and Medicaid," McConnell said. "That's 70 percent of what we spend every year."

McConnell said there was "bipartisan reluctance" to take on entitlement reforms because of their popularity.

"Hopefully, at some point here, we'll get serious about this. We haven't been yet," he said.

McConnell said there was a missed opportunity to tackle entitlements during the Barack Obama administration, owing to the divided government for six of the eight years he held office. Republican President Ronald Reagan and Democratic Speaker of the House Tip O'Neill made a deal in 1983 to raise the Social Security age.

The Republican-controlled government saw spending climb in 2018 due mainly to a hike in military spending, as well as increases for entitlements and disaster relief.

Government revenues were up only $14 billion from last year after Republicans passed a sweeping tax overhaul that included a cut of the corporate rate.

White House Budget Director Mick Mulvaney, mirroring GOP lawmakers last year selling their tax cut plan, said in a statement that America's "booming economy will create increased government revenues."

"But this fiscal picture is a blunt warning to Congress of the dire consequences of irresponsible and unnecessary spending," he said. "Going forward, President Trump and this Administration will continue to work with Congress to make the difficult choices needed to bring fiscal restraint, which, when matched with increasing revenue, will reduce our deficit."