Is Presidential refusal to prosecute millions of fraudulent voters an impeachable offense?

President Trump’s Press Secretary, Sean Spicer, again today claimed there was widespread voter fraud in the most recent election. I don’t think the statute of limitations has run yet, and there are applicable federal voting laws. So if President Trump, who now heads the Executive branch and is therefore responsible for the enforcement of federal laws, refuses to prosecute millions of fraudulent voters, might he not be subject to impeachment?