Media playback is unsupported on your device Media caption Republican Senator Jeff Flake announces his retirement

Shortly after Jeff Flake delivered his blistering speech about Donald Trump from the Senate floor, as media talking heads were swooning and fellow senators were lauding their colleague, the White House had a message for the departing Arizonan.

"Based on previous statements and certainly based on the lack of support that he has from the people of Arizona, it's probably a good move," press secretary Sarah Huckabee Sanders told reporters.

In other words, don't let the door hit you on the way out, Jeff.

Your poll numbers were dismal, and you were already heading for defeat at the hands of a Trumpist firebrand in the primary - an ignominious fate for a man who has been in Congress for 17 years.

And so the battle lines in the Republican Party have been drawn.

Media playback is unsupported on your device Media caption Lashing out: What Bob Corker really thinks of President Trump

On one side is the president, former top adviser and campaign chair Steve Bannon and the rest of the Trump legions, who have embraced his populist, nationalist brand of politics, leavened with a confrontational, take-no-prisoners style of rhetoric.

Mr Trump, the titular leader of the party, hasn't been reluctant to push back against Mr Flake's past criticisms, even meeting the senator's potential primary challengers the last time he was in Arizona.

Mr Bannon already had Mr Flake targeted for defeat.

The Breitbart News chief has already helped take down Alabama Senator Luther Strange, and has virtually every Republican senator running for re-election in his crosshairs.

On the other side are those like Mr Flake, fellow Arizona Senator John McCain, Tennessee Senator Bob Corker and former President George W Bush, who have loudly condemned what they view as the corrosive effect Donald Trump is having on American politics.

Second Republican senator blasts Trump

"Reckless, outrageous, and undignified behaviour has become excused and countenanced as 'telling it like it is', when it is actually just reckless, outrageous, and undignified," Mr Flake said in his speech announcing he would not seek re-election in 2018.

Media playback is unsupported on your device Media caption George W Bush decries bigotry and conspiracy theories

The battlefield is the heart and soul of Republicanism.

Should it be the party of small government, low taxes, free trade, an internationalist foreign policy and conservative social values?

Or is it one of ethno-nationalist sentiment, closed borders, unilateralist foreign policy, America-first protectionism and, well, conservative social values?

The stage is set for a showdown, but at least for the moment the most vocal anti-Trump politicians are heading for the exits, if they aren't there already. Senators like Mr Flake and Mr Corker, in effect, are acknowledging that the party already belongs to the Trumps and the Bannons, rather than running for re-election and proving them wrong.

Most Republican officeholders, who have largely taken the plunge with Mr Trump so far, have stayed in line. And while they tip a hat to Mr Flake, they seem unlikely to follow his words of warning on Tuesday.

"When we remain silent and fail to act when we know that that silence and inaction is the wrong thing to do - because of political considerations, because we might make enemies, because we might alienate the base, because we might provoke a primary challenge, because ad infinitum, ad nauseam," Mr Flake said, "when we succumb to those considerations in spite of what should be greater considerations and imperatives in defence of the institutions of our liberty, then we dishonour our principles and forsake our obligations."

That's not to say there hasn't been effective resistance to Mr Trump within Republican ranks. It's just been of the quiet variety, from senators like Susan Collins of Maine and Lisa Murkowski of Alaska, who have held their tongues while casting votes to sink the president's - and much of their own party's - legislative priorities.

Media playback is unsupported on your device Media caption A protester threw Russian flags at Donald Trump as he headed for a meeting with Republicans

They've found common ground with Democrats on policy matters, and have been less concerned with publicly condemning the president's caustic style. Unlike Mr Flake, their prospects for eventual re-election remain good.

If there's hope for the more conservative anti-Trump forces, it came at the end of Mr Flake's speech, when the first senator to clap - the one whose applause prompted his colleagues to follow suit - was Ben Sasse.

The Nebraska Republican - a former college president with a PhD in history - has himself been an outspoken critic of the president and yet still seemingly harbours long-term political ambitions. He has three years left in his first Senate term, however. He may be hoping the Trumpist surge will be temporary.

That's a view Mr Flake shares.

"This spell will pass," he told the Arizona Republic newspaper.

Perhaps.

If it does, in the end, it will be the ballot box that issues the final verdict. The test will be whether Republican primary voters pick candidates in the Trumpist mould to replace senators like Mr Corker and Mr Flake. And then, in the general election, whether Americans opt for those choices over the Democratic alternative.

One Bannon supporter, feeling triumphant on Tuesday, told the Washington Post his man already has three Senate "scalps", with more to come.

If that's the case, then Trumpism just won another big battle.

Next year, it could win the war.