Gov. Andrew M. Cuomo is spending far more campaign cash — $400,000 per day over the last 21 days — to defeat a progressive challenger in the Democratic primary than he did in 2014, a sign he is taking Cynthia Nixon seriously.

Cuomo spent $8.5 million in the last three weeks alone in his bid to win a third term, according to campaign records published Wednesday by the state Board of Elections. That’s more than three times the amount he spent over the same time period in 2014, when he faced a challenge from Zephyr Teachout, then an unknown Fordham University professor.

In contrast, Nixon spent just a little bit more — $450,000 — over the last three weeks as Cuomo has spent per day.

Since July, Cuomo has spent a cumulative $16 million on the race.

As the Cuomo-Nixon race moves into the final week, the governor has opened the purse strings. Since mid-August, his campaign spent $3 million on television ads and transferred another $2.5 million to the New York State Democratic Committee, which has been buying ads for Cuomo and other candidates.

A Cuomo aide pointed out that some of the recent campaign ads have been focused on Republican candidate Marc Molinaro, not Nixon.

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Cuomo also spent $104,500 on polls in the last three weeks.

The statewide primary is Sept. 13.