The thorny issue of immigration has divided the Republican party more than any other in recent history.
To get a flavour of how sharp the debate has become, just listen to that traditional barometer of grassroots conservative thinking - the radio talk show.
Immigration has brought tens of thousands of demonstrators out
One phone-in host, referring to President George W Bush's primetime television address proposing immigration reforms, described it colourfully as "Like a mackerel in the moonlight. The closer you get to it, the worse it smells."
His views reflect a deep scepticism not from the far left, but from within the Republican party's core supporters.
The problem for the president is that the Republican divide is far from superficial and could get worse.
The immigration debate has highlighted a long-standing fissure between the big-business wing of his party and its socially-conservative populist base.
President Bush's past genius has been to hold these two groups together, preserving an unbeatable Republican majority.
If they fall out - as they could over immigration - then it raises the real possibility of the Republicans losing control of Congress in November's mid-term elections.
Dispute goes public
The two sides are deeply divided.
The country-club business elite is interested in a large supply of cheap labour and support more or less unfettered immigration.
The talk-show populist base is concerned about lawlessness on the border, the strain on public services and competition for jobs.
Social conservatives emphasise the rule of law
The divide is nothing new. What is new is that both camps are beginning to attack each other in public.
In a recent newspaper column, former Republican National Committee chairman Ed Gillespie warned the populists to drop "border enforcement first" legislation.
"Anti-immigration rhetoric is a political siren song, and Republicans must resist its lure," he said.
Another leading country-clubber, Republican commentator Bill Kristol, attacked the talk-show brigade for not recognising the danger of "turning the Republican party into an anti-immigration, Know-nothing party."
History repeating itself?
The reference to "Know-nothing" is no accident. It refers to an extraordinary historical parallel.
It was the name given to a movement that split and almost destroyed the nascent American Republican party 160 years ago.
Then, as today, immigration was the hot issue - although an influx of Roman Catholic immigrants was the big concern.
George W Bush is trying to push the two sides together
The 19th-Century equivalent of the talk-show hosts formed local societies to combat "foreign influences", uphold the "American Way", and restrict citizenship to native-born Americans.
When party officials tried to find and discipline those behind the movement, secretive members would only reply that they knew nothing - hence the name.
Today the populist wing of the party is far from secretive and finds criticism from the country-clubbers deeply offensive.
Far from being driven by xenophobia and intolerance, they consider themselves motivated by a profound respect for the rule of law and a patriotic regard for America's sovereignty and national security.
Now their patience is wearing thin.
In addition to immigration, resentment has been growing on a range of different issues, from the growth of government spending to prescription drug benefits and the series of political corruption scandals.
All drive a wedge between the two Republican traditions.
Soul at stake
So President Bush is walking a delicate tight-rope as he tries to fashion immigration reform that is "comprehensive".
The C-word is code for the pact he is trying to negotiate between the two wings of his party.
In essence, it promises: "We'll be really tough with border enforcement so long as you agree to a guest-worker programme and a path to citizenship for illegal immigrants."
The signs are that the Senate - home to most of the country-clubbers - will sign up to its side of the deal.
The much harder battle will getting agreement in the House of Representatives over the next fortnight.
They face re-election every two years and are closely attuned to the populist concerns of talk radio.
The debate is a historic one.
It could determine the future for millions of illegal immigrants and their families and it will influence whether President Bush retains control of Congress in November's elections - but it also amounts to a fight over the soul of the Republican party.