"Send her back" is the new "lock her up".

Having spent the best part of a week whipping up his base with racist language, President Donald Trump is trying to put the genie back into the bottle.

"I didn't say that, they did. I disagree with it," he said, the morning after the crowd chanted "send her back, send her back, send her back" at a campaign rally in Greenville, North Carolina.

But of course it was Mr Trump who suggested progressive Democratic congresswomen should "go back to where they came from".

The crowd simply took his lead.

Trump supporters gathered for his rally in Greenville, North Carolina this week. ( Reuters: Kevin Lamarque )

For those of us who covered the 2016 campaign it was eerily reminiscent of the chants of "lock her up" that Mr Trump's supporters once directed at Hillary Clinton over her handling of emails when she was secretary of state.

That was an indiscretion that Mr Trump sought to capitalise on politically throughout the campaign.

This time, with no clear individual opponent amid a wide field of Democrats, he has chosen to tackle the party's ethos as it swings left and diversifies.

He is framing himself as the only thing standing between America and the radical hard left.

Who is in his sights?

The self-described "squad" of Democratic congresswomen of colour. ( ABC News )

Mr Trump is targeting Muslim, Somali-born, former refugee and now congresswoman Ilhan Omar and the three other young, progressive women who make up "the squad".

Why don't they "go back and help fix the totally broken and crime infested places from which they came?" the President tweeted about the four congresswomen on the weekend.

All four are US citizens.

The Democratic 'squad' New York congresswoman Alexandria Ocasio-Cortez is of Puerto Rican descent

is of Puerto Rican descent Massachusetts congresswoman Ayanna Pressley is of African-American descent

is of African-American descent Michigan congresswoman Rashida Tlaib is of Palestinian descent

is of Palestinian descent Minnesota congresswoman Ilhan Omar was born in Somalia and emigrated to the US at age 12 after spending much of her childhood in a Kenyan refugee camp

Three were born in the US. One is Hispanic with roots in the US territory of Puerto Rico, one is African-American, two are Muslim.

They were elected at the 2018 midterms, which swept in a historically diverse House.

When asked why he didn't stop the crowd from chanting "send her back" about Ilhan Omar, Mr Trump responded:

"I think I did," he answered. "I started speaking quickly."

In fact, he stepped back from the podium and let the chants go on for 13 seconds before picking up his speech again.

It's less a dog whistle than blatant political strategy, and it resonates with many in a country where racial divides run deep, and where immigration is a top political issue.

Still, many Republicans find Mr Trump's comments this week incredibly discomforting.

And Democrats are deeply affronted, while also milking the moment politically, as some pundits point out.

"This is not about me. This is about us fighting for what this country truly should be," Ms Omar told reporters today.

"This is supposed to be a country where we allow democratic debate and dissent to take place."

Comments sent Americans into debate over what's racist

In disavowing the rally chant, Mr Trump tripled down on his claim that his original tweet wasn't racist, but ideological.

But the phrase "go back to where you came from" has long been associated with race.

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As one Washington Post writer put it, "'go home' rhetoric is as American as immigration itself".

It was used as early as the 1820s, when white preachers urged freed blacks to move to West Africa, saying it would be for their own good.

William Darity, a professor of public policy at Duke University, said the phrase is "invariably a reaction to whatever group is perceived as taking an ideological stance against the status quo".

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He said it would have been one thing had Mr Trump told the women to move to a country associated with socialism.

"If you're telling people to go to Russia, it's not necessarily racist," Professor Darity said.

"You're not telling them to go there because of their race or ethnicity, you're telling them to go there because of their ideology."

Politicians solidified stances with a vote

The heated debate over Mr Trump's tweets had US politicians under pressure to respond.

On Tuesday, the Democrat-controlled House of Representatives passed a formal resolution to condemn Mr Trump's words as racist.

The vote, 240-187, fell almost entirely along party lines. Four Republicans sided with the Democrats, and the pre-vote debate descended into a bitter display of partisanship.

A handful of Republicans defended the President, saying he was not a racist.

A dozen more cast the resolution as a politically fuelled distraction from the real issues.

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"This resolution divides our nation and distracts us from addressing the issues the American people sent us here to solve," said Republican Whip Steve Scalise.

Others went further, saying Mr Trump's words were a correct response to the squad's progressive views.

"I think the president is onto something," said Senate Majority Leader Mitch McConnell.

"We're in a big debate now and next year about what we want America to be like. Do we really think socialism applies here at a time of great prosperity, 50-year-low unemployment?"

But when it came to the chant, the party was less resolute.

A handful of Republicans took a few steps back after the president also suddenly appeared less than gung ho.

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Democrats say Trump is racist, but are divided

The most senior Democrat, Nancy Pelosi, is resisting pressure from within her party to impeach President Trump. ( Reuters: Joshua Roberts )

If the Republican party is starting to show a few cracks over which racist comments to accept, the Democrat party is teetering over a fault line on how to deal with them.

On Wednesday, Representative Al Green of Texas muscled an impeachment resolution onto the House floor.

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It's a move that leaders of the Democratic Party, particularly House Speaker Nancy Pelosi, have actively resisted for months.

She resisted moves to impeach Mr Trump after their midterms election victory, after the publication of the Mueller report, after speculation that he violated the emoluments clause by accepting payments from foreign dignitaries and so on and so forth.

Wednesday's vote ended with the resolution being shelved, but it revealed that a big block of Democrats were against Ms Pelosi's strategy.

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Ninety of the House Democrats' 235 members — including all four members of the squad — voted in favour of advancing impeachment proceedings.

Why are the majority of Democrats against impeachment?

Democrats are divided on whether they should trump to impeach Donald Trump. ( Reuters: Kevin Lamarque )

The 60 per cent of Democrats who voted against the impeachment kick starter are the moderates who are sticking to a poll-tested strategy.

For them, the only way to keep control over the House and have a valid shot at the presidency is to use the strategy that follows precedent: focus on real policy, especially issues like restructuring health care and raising the minimum wage.

The other Democrats — the progressives — say the only way to beat the unconventional President is to fight back, frequently and aggressively, against Trumpism.

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If the unpredictability of the 2016 election is any measure, it would be tough to say which Democratic ideology will get the last word.

With Mr Trump's ability to set an entire week's news agenda with a single tweet, it may well be that neither side of the Democratic schism is loud enough to win — even when the battle lines are being drawn around race.