Deficit vs. debt-to-GDP ratio.

The heavyweight fight of post-budget talking points has been brewing for months.

Once their campaign pledge to keep deficits to a "modest" $10 billion was abandoned, the Liberals shifted to underline another campaign promise: keeping the debt-to-GDP ratio on a downward trajectory.

"In every year of our plan, federal debt-to-GDP will continue to fall... In 2019-20, we will: reduce the federal debt-to-GDP ratio to 27 per cent," read the party's platform.

As he was confessing last month that the deficit in his first budget would be much larger than what was promised on the campaign trail, Finance Minister Bill Morneau fell back on the debt-to-GDP pledge. "We will have a situation where we will be able to reduce our net debt-to-GDP situation during the course of our mandate," Morneau said in French.

Morneau blames deficit on lower oil price3:08

The Conservatives have zeroed in on the admission of a larger deficit and on speculation that it could reach $30 billion or more.

"All that borrowed money is going to have to be paid back," interim Conservative leader Rona Ambrose said last week.

"We will be pushing the federal government to live within its means."

So, is one a better indicator of fiscal health than the other? Debt-to-GDP ratio or deficit?

It's not that simple.

The deficit figure fails to capture how the economy is doing, the size of the country's debt or the impact of interest rates.

But the government has only so much control over the factors that go into determining the debt-to-GDP ratio. It ultimately decides how much money it piles onto the country's debt by running a deficit (or pays down by running a surplus), but it doesn't directly control how much the economy grows or what interest rates are.

Politicians will be using both measures in their talking points over the coming days. The truth is both factors, taken in isolation, don't tell the whole story.

How big will the Liberal deficit be?1:15


How to follow CBC News' budget day coverage

2 p.m. ET: Watch Power & Politics' pre-budget special starting at 2 p.m. ET on CBC News Network and livestreaming on cbcnews.ca and join in a pre-budget live chat at cbc.ca/politics.

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Minister of Finance Bill Morneau will table his first budget in the House of Commons at 4 p.m. ET Tuesday. (Sean Kilpatrick/Canadian Press)

4 p.m. ET: Watch CBC News budget special, hosted by Peter Mansbridge with Rosemary Barton, on CBC Television, CBC News Network and on cbcnews.ca. You can also watch a separate livestream of Finance Minister Bill Morneau's budget speech on cbcnews.ca. And you can listen to CBC Radio's budget special, hosted by Susan Bonner with Chris Hall, on CBC Radio One, 4 p.m. to 5 p.m. ET.

5 p.m. ET: Coverage continues with Power & Politics with Rosemary Barton on CBC News Network and cbcnews.ca. Have a question about the budget and what it means for you? Join our live chat with budget experts at cbc.ca/politics.

7 p.m. ET: Budget coverage continues on The Exchange on CBC News Network.

10 p.m. ET: Peter Mansbridge will wrap up the day's budget coverage with The National at 10 p.m. on CBC Television (9 p.m. ET on CBC News Network).