The first three Republican presidential debates were rough for Rand Paul and even rougher for fans of serious economic discussion. That changed on Tuesday night on Fox Business — on both counts.

In a night filled with substantive questions, Paul forced quite possibly the most memorable — and most critical — exchange: a showdown with fellow Sen. Marco Rubio over taxes, spending and what policies count as "conservative."

The exchange started with Rubio laying out why he has proposed an expanded child tax credit for working families.

You wouldn't know from previous debates, but this is highly controversial among Republican economists. Some of them believe such a credit is a good conservative plan to boost the middle class, and some believe it detracts from the more economically important goal of cutting marginal tax rates — particularly the top rate — as much as possible.

Paul leaped in to attack Rubio on that last point.

The child credit, he said, was "not very conservative." Rubio pushed back, and then Paul went for more: He ripped Rubio for wanting to increase military spending while also cutting taxes.

"How is it conservative to add a trillion dollar expenditure for the federal government that you’re not paying for?" Paul said. "How is it conservative? How is it conservative to add a trillion dollars in military expenditures? You cannot be a conservative if you’re going to keep promoting new programs you’re not going to pay for."

Rubio responded with a security argument: "We can’t even have an economy if we’re not safe," he said. In a moment he added: "I believe the world is a safer — no, I don’t believe, I know the world is a safer and better place when America is the strongest military power in the world."

The discussion moved on soon after, but both candidates had drawn blood.

More importantly, the Republican Party had finally brought one of its most important internal debates into the open. If a GOP candidate wins the White House next year, there will absolutely be a tension in the next Congress over tax cuts, military spending and the federal debt. It's smart for Republicans to start talking about that tension now.