President Trump's intensifying feud with a sitting Republican senator will take center stage Tuesday evening in Phoenix, where Trump will hold a campaign-style rally against the wishes of local authorities and without the attendance of GOP leaders in the state.

The rally comes just a day after Trump's prime-time address about U.S. strategy in Afghanistan won applause from most Republican lawmakers in Washington. But Trump's speeches to his grassroots supporters tend to be less scripted and less predictable, a fact that has some Republicans nervous hours before the president takes the stage in Phoenix.

Trump has turned up the heat on Sen. Jeff Flake as the Arizona Republican barrels toward a tough re-election race in a state the president won last year by nearly 85,000 votes. Trump's crusade against Flake has further shaken his already rocky relationships with Senate Majority Leader Mitch McConnell and other congressional Republicans who have warned the president not to target one of their own in 2018.

Flake will not attend the rally in Phoenix Tuesday evening, his spokesman told the Washington Examiner. Instead, Flake will focus on his "full day of previously-scheduled meetings," including events with Customs and Border Patrol, county sheriffs, and business owners.

The White House did not invite Flake to attend the rally, the Senate aide said.

Trump and Flake have sparred openly since the Arizona Republican declined to endorse his party's nominee during the presidential campaign.

After Flake appeared on several Sunday talk shows in September of last year and expressed doubts about his ability to vote for the GOP candidate come November, Trump took to Twitter to bash Flake as a "very weak and ineffective" leader who has done little to crack down on illegal immigration.

More recently, Trump has openly flirted with the possibility of endorsing Flake's GOP primary opponent, Kelli Ward. After describing Flake as "toxic" in a tweet on Thursday, Trump said it was "great to see" Ward mounting a primary challenge against the one-term senator.

His latest attacks came just a few weeks after Flake penned an op-ed that claimed the Republican Party is "in denial" about the dysfunction of the Trump presidency. Flake's book Conscience of a Conservative: A Rejection of Destructive Politics and A Return to Principle is sharply critical of Trump.

Brad Blakeman, a Republican strategist and former George W. Bush aide, suggested Flake's absence could become a footnote of the evening if Trump keeps the focus on his agenda and not on his feud with both of Arizona's Republican senators. Sen. John McCain, also a frequent Trump critic, is not expected to attend the Phoenix rally because he is undergoing treatment for cancer.

"The fact that Flake and McCain are not present [is] a non-story so long as [Trump] doesn't make it one," Blakeman said.

McCain's vote against the GOP healthcare bill last month prevented Republicans from repealing Obamacare and dealt a serious blow to the president's legislative agenda.

But Trump has so far withheld the withering criticism he typically lavishes on lawmakers who cross him.

"I assume the president will say about Sen. McCain that ‘we all wish him a swift and full recovery, even if he sometimes votes the wrong way.' Attacking a seriously-ill McCain in his home state would be political malpractice," said Charles Lipson, a political science professor at the University of Chicago.

"Attacking Flake would indicate that the President might get involved in a primary there, where Flake will face a pro-Trump candidate," Lipson added.

A McConnell-backed group launched an ad campaign Tuesday against Ward, who ran unsuccessfully for McCain's seat in the 2016 primary. The move was widely perceived as a warning from Senate leadership that the party will marshal resources to defend Flake regardless of Trump's personal problems with him.

Trump has also stoked speculation that he may use the Phoenix rally to announce his pardon of former Maricopa County Sheriff Joe Arpaio, a longtime Trump supporter awaiting sentencing for criminal contempt of court over allegations that he racially profiled people in defiance of a judge's orders.

The president told Fox News earlier this month that he has seriously considered granting the pardon, which would almost certainly touch off a wave of condemnation from Democrats and centrist Republicans. Arpaio's aggressive tactics during his 24-year tenure earned him a reputation as "America's toughest sheriff" but also drew fire from critics who accused his police officers of pursuing and arresting Hispanic citizens simply on the suspicion that were in the country illegally.

Trump's rallies are typically lengthy, unscripted events that draw rave reviews from his core supporters while occasionally igniting controversy when the president veers too far off message.

"He has to lay out his fall agenda and challenge the Congress to act on behalf of the American people," Blakeman said. "He needs to reach out to Democrats as well. 2017 must be the year for action because 2018 will be a year of reflections the American people go to the polls for the midterm elections."

When lawmakers return from their August recess at the end of the month, they must quickly pass a budget and address the debt ceiling before they can move on to other items Trump has prioritized this year, such as healthcare and tax reform.

Trump's widely-panned response to racial violence in Charlottesville, Va., earlier this month sapped much of his political capital among congressional Republicans and sowed doubts about his ability to stabilize what has recently become a chaotic administration.

Although Trump turned the page on Monday with a formal address to the nation about the war in Afghanistan — which he began by calling for national unity — he could easily slip back into the controversy during the Phoenix rally if he made more indelicate comments about the white supremacist groups that marched in Virginia on Aug. 12.

"The [president] needs to amplify the need to unify as he alluded to last night," Blakeman said.