WASHINGTON — Retired teacher Dan J. Sullivan, also known as “Decoy Dan,” has been accused by a watchdog of secretly and illegally colluding with a liberal group to take down his namesake Alaska senator.

The connection comes from veteran Democratic strategist Amber Lee, who worked with his campaign while sitting on the board of the left-leaning 907 Initiative, a government transparency watchdog that has spent some $1.5 million on ads attacking incumbent Sen. Dan S. Sullivan (R-Alaska), disclosures show.

“It is absolutely a violation of federal election law for someone to wear two hats,” Caitlin Sutherland, the executive director of watchdog group Americans for Public Trust, which filed the complaint Tuesday, told The Post.

“You can’t both work for the campaign and work for the outside group that’s running millions of dollars in advertisements that’s benefiting the campaign that you’re working for.”

About $575,000 of 907 Initiative’s spending against Sen. Sullivan took place during the final 90 days of the primary race, APT alleged in a complaint filed with the Federal Election Commission Tuesday. “Decoy Dan” jumped into the race on May 29.

Just one day before “Decoy Dan” officially filed, the 907 Initiative slammed Sen. Sullivan and launched an ad titled “Stop Raising Costs.” His campaign owed Amber Lee Strategies $4,380.41, the complaint noted.

“Amber Lee Strategies’ work for the Daniel J. Sullivan campaign — and her concurrent leadership position as a board member of 907 Initiative whilst it spent significant sums of money on ads targeting her client’s primary opponent — is the third and final prong to establish coordination,” APT wrote in its complaint.

“In fact, Ms. Lee helped Daniel J. Sullivan file to run and ‘helped author some of Dan J. Sullivan’s campaign materials’ at a time when her 907 Initiative was already reserving advertisements attacking his opponent,” the complaint went on.

Lee’s assistance to “Decoy Dan” was revealed in an August report by the New York Times, though he indicated to the outlet that the two had since cut ties.

There’s also a reported federal probe into his campaign to determine whether there was a conspiracy to confuse voters by getting a challenger with the same name as the senator on the ballot to help Democrat Mary Peltola.

“Sometimes when individuals do engage in schemes, it can be a little bit difficult to see what is happening. This is just a very clear example of coordination,” Sutherland said.

Shortly after “Decoy Dan’s” entrance into the critical Alaska Senate race, he faced accusations of mounting a bad-faith campaign to deliberately confuse voters because of his identical name to Sen. Sullivan.

Despite working with a Democratic strategist, “Decoy Dan” registered as a Republican — and doing so just days before filing his candidacy.

Alaska Division of Elections Director Carol Beecher cited that in her initial decision to kick him off the ballot, after concluding he was running a bad-faith campaign aimed at tricking GOP voters. The courts have since let him back on the ballot, and he advanced to the general election last month.

She noted that the color scheme of “Decoy Dan’s” initial campaign website looked remarkably similar to that of Sen. Sullivan. Beecher also drew attention to the fact that he used Sen. Sullivan’s initial in a private email to her, though he later corrected it and claims to have filed his paperwork with the right initial.

“This allegation is patently false,” 907 Initiative executive director Aubrey Wieber told The Post. “907 Initiative has never employed or paid Amber Lee. Amber Lee is a board member of 907 Initiative. The fact that Americans for Public Trust can’t decipher the difference between those two tells you all you need to know.”

“907 Initiative had no involvement with any work between Dan J. Sullivan and Amber Lee. I found out about the connection by reading the news.”

The retired public teacher claimed that he is a moderate Republican, and admitted to the New York Times that he voted Democratic in the past three presidential cycles, participated in a “No Kings” protest, and supported Peltola in both her congressional campaigns.

Sen. Sullivan’s showdown against Peltola is one of the most competitive Senate races in the country. During the jungle primary last month, Peltola notched 49.5% of the vote compared to Sen. Sullivan’s 41.4%. Alaska has everyone face off in a primary, and the top four candidates advance to the general.

Peltola is ahead of the Alaska senator by 2 percentage points in the latest RealClearPolitics polling aggregate.

APL is asking the FEC to launch a probe into whether “Decoy Dan” and the 907 Initiative illegally coordinated through Lee.