ST. LOUIS • In 2001, Mayor Francis Slay took over a city that had been hemorrhaging population for decades.

Between 1980 and 2000, more than one out of every five residents fled the city limits – a loss of 104,000 thousand people.

In his two and half terms as mayor, almost every major imitative Slay has invested in – improving education, building up the Loft District, stabilizing neighborhoods – has revolved around reversing the population exodus that has hamstrung St. Louis since the middle of the last century.

Today is Slay's day of truth.

While the mayor's office has produced elaborate appeals in the intervening years that suggest the city has added residents, those only counted for unofficial census estimates.

The release of the decennial census this afternoon will stand as the city's official population for the next ten years, and either an affirmation – or repudiation – of Slay's policies.

More than that, the census results will go a long way towards defining Slay's legacy. He's already one of just a handful of mayors elected three times. Will Slay be the first since mayor since Aloys P. Kaufmann, elected during World War II, who will be able to claim the city population grew under his watch?

Should the census show a population loss for the city, there are some ways Slay can claim partial victory – say, if the decrease is not as large as previous years, or if the percentage is lower than the overall population decline in the state or region.

Even so, being able to take credit for the city's growth is a lot more desirable than settling for a slower rate of decline.

Whatever today's numbers show, the mayor's office won't be the only one in City Hall taking notice. Slay's aides are operating under the assumption that he will seek an unprecedented fourth term.

If the city does not show a population gain, it may embolden some potential Slay opponents in 2013 – who won't have to look far for their first talking points.