Conway, Trump's campaign manager during the stretch run of the 2016 race, was the first woman to successfully steer a White House bid, then became a senior counsellor to the president.

She informed Trump of her decision in the Oval Office.

Conway cited a need to spend time with her four children in a resignation letter she posted on Sunday night. Her husband George had become an outspoken Trump critic and her family a subject of Washington's rumour mill.

"We disagree about plenty but we are united on what matters most: the kids," she wrote.

"For now, and for my beloved children, it will be less drama, more mama."

She is still slated to speak at the Republican National Convention this week. Her husband, a lawyer who renounced Trump after the 2016 campaign, had become a member of the Lincoln Project, an outside group of Republicans devoted to defeating Trump.

The politically adversarial marriage generated much speculation in political circles and online. George Conway also announced he was taking a leave of absence from Twitter and the Lincoln Project.

Kellyanne Conway worked for years as a Republican pollster and operative and originally supported senator Ted Cruz in the 2016 Republican primary. She moved over to the Trump campaign and that August became campaign manager as Stephen Bannon became campaign chairman. Bannon was indicted two days ago for fraud.

Conway cited a need to help her children"s remote learning during the coronavirus pandemic as a need to step away from her position.

She was known for her robust defence of the president in media appearances, at times delivering dizzying rebuttals while once extolling the virtues of "alternative facts" to support her case.

Conway was also an informal adviser to the president's re-election effort but resisted moving over to the campaign.

Her departure comes at an inopportune time for Trump, who faces a deficit in the polls as the Republican National Convention begins on Monday.

Australian Associated Press