A letter would represent the clearest sign to date that Trump, front-runner for the 2024 Republican presidential nomination, may face federal criminal charges around his efforts to remain in power after losing the election to Joe Biden.

Officials have testified that during his final months in office, Trump pressured them with unsubstantiated claims of widespread voter fraud. His supporters attacked the U.S. Capitol in a Jan. 6, 2021, bid to stop Congress from certifying Biden's win.

Smith "sent a letter (again, it was Sunday night!) stating that I am a TARGET of the January 6th Grand Jury investigation," Trump wrote on his Truth Social media site.

His attorneys could not immediately be reached for comment.

NBC News, citing law enforcement sources, confirmed that Smith had sent the letter. A spokesperson for Smith's office declined to comment.

Trump's legal woes have so far failed to dent his efforts to win the 2024 Republican nomination. His lead in opinion polls has widened in recent months and his Republican rivals have largely supported him in the face of several criminal probes.

Peter Zeidenberg, a former federal prosecutor, said receiving a target letter means the individual should "presume that you're going to get indicted, unless you give us a damn good reason why we shouldn't."

Smith has already brought criminal charges against Trump in a separate federal case alleging he unlawfully retained national security documents after leaving office in 2021 and conspired to obstruct efforts to retrieve them. Trump has pleaded not guilty.

According to a court filing, Smith sent a target letter to Trump's lawyers in that case on May 19, roughly three weeks before he filed charges.

Defense attorneys and federal prosecutors were due to make their first appearance in Florida later on Tuesday before U.S. District Judge Aileen Cannon, who is presiding over that case.

As part of the 2020 election investigation, a grand jury at the federal courthouse in Washington, D.C., has heard testimony from high-profile former Trump administration officials, ex-White House attorney Pat Cipollone and Trump's then-Vice President Mike Pence.

Most recently, Trump's former attorney Rudy Giuliani was interviewed by investigators from Smith's office.

In his Truth Social post, Trump said Smith's office had given him "a very short 4 days" to appear before the grand jury in the probe.

In federal investigations, targets who are not called to testify before a grand jury or who do not make such a request are sometimes provided an opportunity to do so before prosecutors seek an indictment.

In the Jan. 6, 2021, rampage at the Capitol, Trump's supporters used weapons including chemical sprays and riot shields to attack police and break into the building, forcing lawmakers who were in the process of certifying the results of the 2020 election to flee for their lives.

The attack came shortly after Trump told supporters in an incendiary speech near the White House to march on the Capitol and "fight like hell" to "stop the steal" of the election.

More than 1,000 people have been charged with crimes connected with the riot, including some who have been convicted of seditious conspiracy.

Florida Governor Ron DeSantis, Trump's leading rival for the Republican nomination, said Trump should have "come out more forcefully" to deter the Jan. 6 attack, but questioned whether that should lead to criminal charges.

"We (don't) want to be in a situation where, you know, you don’t have one side just constantly trying to put the other side in jail. And that unfortunately is what we’re seeing now," he said at a campaign stop in South Carolina.

Trump is the first former president ever to be indicted.

In addition to the charges in the classified documents case, Trump faces New York state criminal charges accusing him of falsifying business records concerning a payment to porn star Stormy Daniels to buy her silence before the 2016 election about a sexual encounter she has confirmed but he denies.

He has pleaded not guilty in that case as well.

On Monday, the Georgia Supreme Court rejected a bid by Trump to block a state investigation into whether he and his allies illegally attempted to meddle with Georgia's 2020 election.

Among actions being investigated was Trump's phone call urging a top election official to "find" enough votes to overturn Biden's statewide victory.

Republican strategist Matt Dole said any indictment would likely help Trump with fundraising but could deter some moderate Republicans and hinder his chances in the general election.

"I do think that it gives soft Trump supporters pause and reason to look elsewhere for a candidate with Trump's populism, but less of the circus - someone like Ron DeSantis," Dole said.