Two Colorado presidential electors filed a lawsuit Tuesday in federal court challenging a state law that requires them to vote for the winner of the state’s popular vote.

The move is part of the long-shot effort to use the Electoral College vote to block Donald Trump from winning the presidency.

Two Democratic electors who pledged to support Democrat Hillary Clinton — the winner of the state’s nine Electoral College votes — now want to “vote their conscience and do their constitutional duty as intended by the framers,” said Jason Wesoky, the attorney who filed the suit.

Polly Baca, a former state lawmaker, and Robert Nemanich are among the “Moral Electors” hoping to persuade Republican electors in other states to vote for a third-party candidate to keep Trump from receiving 270 electoral votes — and offering to shift their Democratic votes to a consensus pick.

Another Colorado elector, Micheal Baca, who is not part of the lawsuit, is working with electors in other states to help push what they call the “Moral Electors” movement.

Colorado law prohibits them from shifting their allegiances, and Republican Secretary of State Wayne Williams has pledged to replace them if they do so — two moves the electors argue violate the U.S. Constitution.

“Though Hillary Clinton and Timothy Kaine won the majority vote in Colorado and are qualified for office, plaintiffs cannot be constitutionally compelled to vote for them,” the lawsuit states. “Plaintiffs are entitled to exercise their judgment and free will to vote for whomever they believe to be the most qualified and fit for the offices of president and vice president, whether those candidates are Democrats, Republicans or from a third-party.”

Williams responded with an uncharacteristically sharp retort, vowing to defend Colorado’s law and suggesting that the electors “succumbed to cabal, intrigue and corruption.”

“Instead of honoring the will of the Coloradans who voted for them, these two faithless electors seek to conspire with electors from other states to elect a president who did not receive a single vote in November,” Williams said in a statement.

He continued: “Make no mistake, this is not some noble effort to fight some unjust or unconstitutional law; rather, this is an arrogant attempt by two faithless electors to elevate their personal desires over the entire will of the people of Colorado. And in so doing, they seek to violate Colorado law and their own pledges.

“The very notion of two Colorado electors ignoring Colorado’s popular vote in an effort to sell their vote to electors in other states is odious to everything we hold dear about the right to vote,” Williams said. “It is this type of evil that President Franklin Roosevelt warned us about when he cautioned that voters — not elected officials such as these faithless electors – are ‘the ultimate rulers of our democracy.’”

One potential candidate as an alternative to Trump is Ohio Gov. John Kasich. But he issued a statement Tuesday telling electors not to vote for him.

The effort to prevent Trump from taking the White House is not likely to succeed, but the challenge to Colorado’s law could prove more interesting.

The electors believe the state law violates their First Amendment rights as well the 12th and 14th amendments, which respectively outline the process for electing a president and guaranteeing equal protection.

The lawsuit points to the Ray v. Blair case in 1952, where the plaintiffs argue that the U.S. Supreme Court left open the question about whether states can compel electors to vote or impose penalties.

The secretary of state’s office argues that when an elector takes the oath Dec. 19 — when the Electoral College meets — they become public officers. And if they fail to meet their public duty under the law, they could be subject to a misdemeanor criminal charge.

Gov. John Hickenlooper, a Democrat, and Attorney General Cynthia Coffman, a Republican, also are named as defendants in the lawsuit. Both declined to comment.