A fresh battle in the Republican civil war flared Thursday as former President George W. Bush took veiled shots at President Trump in a speech that warned against the rising tide of ethno-nationalism on the Right.

Bush sounded the alarm about "nationalism distorted into nativism," chiding that American identity "is not determined by geography or ethnicity, by soil or blood." He cautioned against the siren song of isolationism. The 43rd president never mentioned Trump.

But in issuing a clarion call for the conservative ideals of Ronald Reagan Republicanism, Bush was unmistakably rebuking the "America First" sloganeering of a 45th president who has questioned the value of U.S. internationalism and been accused of winking at white-identity politics.

"Right now there are at least two Republican parties," GOP strategist Doug Heye said. "We know there are two; there might be more."

It was rare criticism of a successor from Bush and remarkable in that it was directed at a fellow Republican, the first one to hold the White House since he exited nearly nine years ago. Bush spoke in New York, Trump's hometown.

Bush is the latest in wave of prominent establishment Republicans attempting to hold at bay the populism ascendant in the GOP under Trump, even though this president's policies don't always comport with his nationalist rhetoric.

Sen. John McCain, R-Ariz., delivered similar remarks in Philadelphia just days earlier.

He said it was significant to have a former president join him on such an urgent concern. McCain, 81, in Congress since 1983 and his party's 2008 presidential nominee, said never before during his time public life had he worried that Americans might be susceptible to nationalism or isolationism.

"It's already taken off; it's already taken off. What do you mean?" McCain said Thursday in an interview, when asked if he fretted that these ideals were gaining ground in the GOP. "It's isolationism, we're going to throw people out of office, we're going to beat [Sen.] Jeff Flake [R-Ariz.] with a wacko primary opponent…This is all about the future — not just of the Republican Party but of the United States of America."

"I'm a student of history, I know what happened in the 1930s — the ‘America First'ers,' isolationism, rise of Nazism, America withdrawing. That's what happened," McCain added.

In the aftermath of the Great Recession, voters have questioned the benefits of free trade agreements that have contributed to the reduction of domestic manufacturing, and robust immigration and the ensuing competition for labor in some industries.

With the Cold War receding from view, and the threat of Jihadist terrorism and costly wars in Afghanistan and Iraq at the forefront, the dividends of internationalism have seemed less clear.

Trump won the presidency on a campaign fueled by these frustrations and uncertainties. He threatened to cancel trade deals, reduce legal immigration generally and curtail Muslim immigration specifically, and withdraw the U.S. from key international alliances.

The president's actions thus far have been less radical than his rhetoric. He's pushing for a traditional, Republican tax reform plan, has beefed up Washington's overseas commitments and made minimal moves on immigration.

That suggests, say some Republican strategists, that what is really roiling the GOP are stylistic differences. The nationalists might be bigger players in the party's voting coalition, but it hasn't changed all that much.

"The battle is more about civility versus passion, tone and tenor, rather than policy," said David Carney, a Republican strategist in New Hampshire. "Take the personalities out and there is 90 percent agreement of policy objectives."

Trump's populist allies, led by former White House chief strategist Steve Bannon, are intent on making good on the president's nationalist campaign promises.

They're recruiting candidates to primary established Republicans (Flake's GOP opponent, Kelli Ward, is one of them) with the goal of creating a party that is less in the image of former Reagan and more nationalist, reminiscent of President Andrew Jackson.

"Like [Andrew] Jackson's populism, we're going to build an entirely new political movement," Bannon, who runs Breitbart News, told the Hollywood Reporter late last year, soon after Trump was elected. "It's everything related to jobs. The conservatives are going to go crazy. I'm the guy pushing a trillion-dollar infrastructure plan."

Republicans fighting these changes worry that the nationalists will usher in a Republican Party that is less tolerant and welcoming, of immigrants, nonwhites and Muslims and others from a non-Judeo Christian background.

Bush, in office during the Sept. 11, 2001, terrorist attacks on New York, Washington and Pennsylvania that were committed by radical Islamists, alluded to this on Thursday when he said that "people of every race, religion, and ethnicity can be fully and equally American. It means that bigotry or white supremacy in any form is blasphemy against the American creed."

Trump's decision to revive the "America First" slogan prominent among U.S. isolationists in the 1930s who urged against entering World War II and fighting Nazi Germany, and adopt it as his own, has led to lingering anxiety about the president's ideas for America's place in the world.

"McCain and Bush are pushing back on some very dangerous forces that are threatening the conservative movement," GOP strategist Alex Conant said. "The two don't always see eye to eye, so it speaks volumes that they're both speaking out now."