Former national security adviser John Bolton John BoltonJudge appears skeptical of Bolton's defense of publishing book without White House approval Maximum pressure is keeping US troops in Iraq and Syria Woodward book trails Bolton, Mary Trump in first-week sales MORE has returned to United Against Nuclear Iran (UANI), the advocacy group announced Tuesday.

Bolton had served on UANI’s advisory board until 2018, when he became national security adviser for President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE.

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UANI describes itself as a "bipartisan, not-for-profit advocacy group leading efforts to warn companies and governments around the world about the dangers posed by the Iranian regime."

In a press release, Bolton said, "Iran poses one of the most serious and sustained threats to America and her allies.

"I’m looking forward to returning to UANI — an organization that has shown tremendous results over the past few years — because it is solely focused on addressing the Iranian threat and it does so successfully."

Bolton left the White House in September, but his name has come up multiple times in House Democrats' ongoing impeachment investigation into Trump.

However, Bolton declined earlier in the month to voluntarily testify to the House Intelligence Committee, stating that he would only testify if the committee issued him a subpoena.

So far, the committee has yet to subpoena Bolton.