President Donald Trump’s new executive order on mail-in voting is offering the clearest explanation yet for why the Justice Department has spent months aggressively seeking access to state voter rolls, while also triggering a sweeping legal challenge from Democrat-led states.

Twenty-three Democratic states and the District of Columbia filed a lawsuit on Friday, just days after Trump signed the order, in federal court in Massachusetts seeking to stop the Trump administration from implementing it, arguing it unconstitutionally interferes with states’ authority to run elections. The lawsuit, which is led by California, contends that neither the Constitution nor federal law gives the president power to mandate changes to state election systems.