Former President Donald J. Trump and House Speaker Mike Johnson are expected to make a joint announcement on Friday about “election integrity,” a broad term often used by Mr. Trump and other Republicans to cast doubt on elections the party lost.

The remarks, scheduled to take place at Mar-a-Lago, Mr. Trump’s private club and residence in Palm Beach, Fla., will come as Mr. Johnson faces criticism from the right over his handling of issues such as aid to Ukraine, and a threat to his speakership from a top Trump ally, Representative Marjorie Taylor Greene of Georgia.

Neither Mr. Trump, the presumptive Republican nominee for president, nor Mr. Johnson has publicly elaborated on the subject of the announcement, but the former president has made his false claims that the 2020 election was stolen from him a major focus of his 2024 campaign.

At rallies and campaign events, Mr. Trump continues to declare falsely that he won in 2020, referring to the election as “rigged” or “stolen.” He repeatedly pushes baseless assertions of voter fraud and, without evidence, accuses Democrats of cheating.

In a statement released by the Biden campaign, Representative Bennie Thompson, a Democrat of Mississippi, called the expected announcement a “sham” and criticized Mr. Trump for repeating his election lies.

“Donald Trump and Mike Johnson don’t care about election integrity,” Mr. Thompson said. “They care only about helping Trump’s campaign of revenge and retribution to regain power at all costs.”

Mr. Johnson played a significant role in supporting Mr. Trump’s false claim that he won in 2020, recruiting House Republicans to sign a legal brief that supported a lawsuit seeking to overturn the election results. He repeated claims about voter fraud in interviews, and he provided Republicans with arguments that some used to object to certifying Joseph R. Biden Jr.’s victory on Jan. 6, 2021.

Well ahead of Election Day this November, Mr. Trump — who faces criminal charges related to his efforts to overturn the 2020 race — has already tried to sow doubt about the 2024 election. He routinely insists at his campaign rallies that Democrats cannot possibly win in November without cheating and has more recently urged his supporters to turn out in droves to ensure that his vote total is “too big to rig.”

As he and his campaign took over the Republican National Committee earlier this year, Mr. Trump backed a new chair, Michael Whatley, who he believed held views about election fraud more in line with his own. Such fraud is exceedingly rare despite Mr. Trump’s insistence to the contrary.

Mr. Trump’s daughter-in-law, Lara Trump, was chosen as the committee’s co-chair. Last month, she told NBC that she believed the party was ready to leave the 2020 election in the “past,” even as Mr. Trump continued to re-litigate it on the stump. And she has said that Republicans need to encourage supporters to vote early where legal, even as her father-in-law denounces the practice.

This year, Mr. Trump has encouraged the Republican National Committee to ramp up its investments in so-called election integrity initiatives, like training poll watchers and filing lawsuits over election procedures both before and after Election Day.

Republicans in key battleground states have also since 2020 pushed for increased restrictions on voting, including laws requiring identification at polling places and more limits on mail-in voting and early voting, practices that have tended to favor Democrats in recent cycles.

Mr. Trump has repeatedly attacked both practices, arguing frequently that mail-in voting is rife with fraud and that elections need to be limited to “one-day voting.” Other prominent Republicans, particularly in battleground states, have said that the party needs to encourage the practice in order to chip away at Democrats’ advantages.

As speaker, Mr. Johnson has made a public show of his continued support for Mr. Trump, even as the former president has undermined some of his legislative efforts. His predecessor, Kevin McCarthy, was ousted after a right-wing rebellion, leading to days of chaos as House Republicans struggled to pick a replacement.