President Joe Biden will issue blunt new warnings about ongoing threats to US democracy in a major address Thursday, sharpening the central argument in his potential rematch with Donald Trump, who he's accused of trying to "destroy" bedrock American institutions.

As indictments and arrests of the former president piled up over the summer, Biden remained mostly silent on his predecessor, wary of appearing to intervene in Justice Department business. His most substantive comment on Trump's myriad legal issues was a sarcastic remark about his mugshot in the Fulton County, Georgia, case.

But as Trump's prohibitive lead in the Republican primary remains unchanged -- and as Biden's own standing remains mired in low approval -- the president is sharpening his attacks on his most likely 2024 rival as a danger to democracy, and allies of the president say that Thursday's speech should serve as yet another sign that the days of trying to keep Trump at an arm's length are long gone.

And if stopping the erosion of democratic institutions and values was central to Biden's decision to run for president in 2020, it will again be core to his reelection campaign, officials said, as he looks to energize voters and donors who have otherwise appeared lukewarm about a rematch between the two men.

"Protecting democracy," a senior White House official said, "continues to be the central cause of Joe Biden's presidency."

Senior Biden advisers have been mulling over the timing and location of Thursday's speech for weeks. Previously, Biden has sought to harness the symbolic settings of Independence Hall and Gettysburg to issue warnings about the state of American democracy.

Advisers eyed similar sites pegged to American history on the East Coast before settling on Tempe, Arizona, in part as a way to honor the late Republican Sen. John McCain, whom Biden was friends with for decades and referred to as a "brother." Arizona was also a center of Trump's efforts to overturn the 2020 election results, and a state where voters rejected candidates who denied the results two years later.

That effort will loom large in the president's message, which is expected to highlight the significance of the Constitution as the bedrock of democracy and the importance of separation of powers and checks and balances, according to a person with knowledge of the speech.

"This one will focus on the importance of America's institutions in preserving our democracy and the need for constant loyalty to the US Constitution," a White House official said ahead of the speech.

They also selected the day after the second Republican primary debate, hoping to insert Biden into a news cycle otherwise dominated by the GOP contest. Trump skipped the debate, delivering a speech in Michigan instead as he looks to cut into Biden's support among union workers.

The speech comes at a moment of political uncertainty for Biden, as he faces persistent questions about his age, disapproval of his handling of the job and an indictment of his son, Hunter. House Republicans plan to hold their first hearing in an impeachment inquiry into Biden on Thursday.

Many senior Democrats believe once voters come to see the 2024 election as a contest between Biden and Trump, the stakes will be clearer and the current president's standing will improve.

Top Biden donors, many of whom have agitated for more forceful attacks on Trump at this early stage in the campaign, were informed of the plans for Thursday's speech by senior Biden advisers during a fundraising retreat in Chicago earlier this month. Biden began previewing his address to donors behind closed doors last week.

In those remarks, Biden has debuted new warnings about his predecessor's potential return to the White House, testing the material off-camera as he and his team were preparing for Thursday's address.

"Let there be no question: Donald Trump and his MAGA Republicans are determined to destroy American democracy. And I will always defend, protect, and fight for our democracy. That's why I running," he said at a Broadway theater last week.

Two days later, he amplified his warnings to a group of lawyers -- and said he was confident he could defeat Trump for a second time.

"I'm now running again. Because guess what? I think that it's likely to be the same fellow, and it's likely that I think I can beat him again," he said.

Defending democracy is an issue Biden allies believe remains deeply resonant with voters, almost three years after the 2020 contest. The video announcing his reelection opened with footage of the January 6, 2021, attack on the US Capitol.

In the lead-up to the 2022 midterm elections, Biden delivered a resounding message in front of Philadelphia's Independence Hall, warning of "MAGA forces" that "tried everything last time to nullify the votes of 81 million people." Ahead of the speech, Biden convened his communications staff with a group of academics and historians -- including Pulitzer Prize-winning author Jon Meacham, who has helped draft his highest-profile addresses -- to reflect on the fragile state of the union and compile ideas.

The White House remains in touch with several of those historians to continue generating ideas, but officials did not know whether Biden had hosted a formal gathering to assist in drafting the speech.

But Democrats say the message worked. The administration and national Democrats have touted the results of the 2022 midterm elections, and the fact that a so-called red wave never materialized as many had predicted, as proof the president's focus on themes like defending democracy struck a chord.

The day after election day last year, Biden said at a news conference: "It was a good day, I think, for democracy."

"Our democracy has been tested in recent years, but with their votes, the American people have spoken and proven once again that democracy is who we are," he said.

As with any major Biden speech, aides expect changes could be made until the final moments. Thursday's remarks are being billed by the White House as the president's fourth major speech on the theme of democracy -- Biden spoke to the issue last year to mark the one-year anniversary of the January 6 insurrection, as well as days before the midterm elections.

In a speech to Democratic donors on Tuesday evening in Silicon Valley, Biden harkened back to Trump's actions during the insurrection attempt -- now the subject of multiple criminal indictments.

"If you ever come to the Oval Office I'll show the dining room where the other guy sat for hours," he claimed.

By also honoring McCain during his speech Thursday, Biden hopes to harken to an era of bipartisanship in Washington that has disappeared in recent years. The comparison will be amplified given the current battle over government funding, which appears destined to result in a government shutdown by the end of the week.

A White House official noted Arizona was a state that has toggled between Democratic and Republican leadership.

"President Biden will talk about his conviction that we must not walk away from the sacrifices generations of Americans have made to defend our democracy," the official said.

He'll be joined at the speech by McCain's widow Cindy, other members of the McCain family and Democratic Gov. Katie Hobbs.

However, one of the state's senators, Kyrsten Sinema -- who was a Democrat until she left the party last year to become an independent -- said Biden should use his visit to Arizona to observe the situation at the southern border.

"It's well past time for President Biden to see the border crisis first hand and for the administration to do its job, secure the border, and keep Arizona safe. While he's in Arizona, I'm calling on him to visit the border to actually understand how our communities shoulder the burden of his administration's failure to address this crisis," she said in a statement.

McCain's death was deeply personal and painful for Biden for a number of reasons, including the fact that McCain had been diagnosed with the same cancer that took the life of Biden's son, Beau. After laying a wreath near the site where McCain's plane was shot down in Hanoi this month, Biden said he missed his former Senate colleague.

"He was a good friend," Biden said.

In his eulogy for McCain in the summer of 2018, Biden described his friend as having "lived by a different code -- an ancient, antiquated code where honor, courage, integrity, duty were alive."