New beginnings are often followed by as many opportunities to succeed as chances to fail. Nothing underscores that in NYC more than Gotham's newest Dynamic Duo: Mayor Bill de Blasio and Council Speaker Melissa Mark-Viverito.

In his first month in office, de Blasio sealed Mark-Viverito's rise, exhorted the virtues of a "national urban consensus," challenged the feds to help him save beleaguered Brooklyn hospitals, and locked horns with Gov. Andrew Cuomo over universal pre-kindergarten.

In her first weeks as speaker, Mark-Viverito doled out plum City Council assignments to supporters, vowed to raise the minimum wage, and made a pathway to citizenship for immigrants here illegally a priority.

The pace has been dizzying, and both now are trying to fast-track a major expansion of the city's sick-leave policy to supersede a law approved last year.

Their effort is as wide as it is consequential: The new measure, to be introduced next week, would terminate exemptions for manufacturing businesses, remove economic benchmarks that would stall implementation if the city economy tanks, and lower the employee threshold from 15 to five. Those employed more than 30 hours a week could earn up to five paid sick days a year.

Employees should not have to choose between a sick day and financial stability when they or a family member become ill.

But what's the hurry? Why impose the additional burden on small neighborhood businesses already operating at tissue-thin margins, with no phase-in?

A smarter approach would let the current law take full effect and study its impact over, say, 12 months. Then, if needed, adjust it based on the new reality.

De Blasio, Mark-Viverito and their supporters point to other cities (San Francisco) and states (Washington) that have adopted new sick-leave requirements, noting scant reports of businesses forced to close or to lay off employees in those jurisdictions. But no two places are wholly the same, so comparisons can be dicey.

Given that Democrats have been out of power at City Hall for more than 20 years, the rush by de Blasio and Mark-Viverito is understandable. They just need to make sure they don't trip over themselves on the way.

Eli Reyes is deputy editorial page editor for amNewYork.