The prosecution's star witness, former Trump attorney Michael Cohen, called today's verdict "an important day for accountability and the rule of law."

"While it has been a difficult journey for me and my family, the truth always matters," he said.

Two senior Trump advisers who ABC News spoke with were extremely angry after the verdict.

Both said they were not surprised by the jury’s decision. One said, "The fix was always in," and the other added, "Not surprised -- it’s New York."

Donald Trump Jr., reacting to his father's guilty verdict, said, "The Democrats have succeeded in their yearslong attempt to turn America into a third-world s---hole. Nov. 5 is our last chance to save it."

The guilty verdict shows "no one is above the law," a representative for President Joe Biden's campaign said, adding, "But today's verdict does not change the fact that the American people face a simple reality. There is still only one way to keep Donald Trump out of the Oval Office: at the ballot box."

"Convicted felon or not, Trump will be the Republican nominee for president," Michael Tyler, communications director for the Biden-Harris campaign, said in a statement.

"A second Trump term means chaos, ripping away Americans' freedoms and fomenting political violence -- and the American people will reject it this November," Tyler said.

The White House offered a very brief comment, with a White House Counsel’s Office spokesperson saying, "We respect the rule of law, and have no additional comment."

Among the congressional response, Speaker of the House Mike Johnson, who traveled to Manhattan days ago to support Trump at the trial, said in a statement, "Today is a shameful day in American history. Democrats cheered as they convicted the leader of the opposing party on ridiculous charges, predicated on the testimony of a disbarred, convicted felon."

"President Trump will rightfully appeal this absurd verdict -- and he WILL WIN!" Johnson said.

Former President Donald Trump's campaign is already fundraising off of the guilty verdicts in his hush money trial, telling his supporters he has been convicted in a "rigged" trial.

"I was just convicted in a RIGGED political Witch Hunt trial: I DID NOTHING WRONG!" a fundraising email said. "They’ve raided my home, arrested me, took my mugshot, AND NOW THEY’VE JUST CONVICTED ME!" the email continues.

The email links to a fundraising page, in which Trump told his supporters, "I'm a political prisoner!"

As ABC News reported previously, the U.S. Secret Service spent considerable time working with New York City court and jail officials to determine what would happen if the judge, as threatened during trial, ordered former President Trump to be incarcerated for contempt -- a situation that did not come to pass.

According to an official briefed on the matter, U.S. Secret Service planners have not started the process of determining arrangements for Trump to be imprisoned at a New York state penitentiary in the event he is convicted. Because Trump is charged with nonviolent crimes, has no criminal record, and was allowed to remain free before trial, it is believed that there is no way that a conviction would lead to immediate incarceration.

If Trump is convicted, the Secret Service will spend the time prior to sentencing -- which could be months -- working with New York state prison officials to determine how imprisonment would work. An official told ABC News that potential questions could include, would USSS agents be armed inside prisons where firearms are prohibited? What prison would Trump be assigned to? Where would protection agents be stationed in relation to Trump? Those details, among many others, would have to be hammered out, the official said.

The Secret Service specifically avoided planning on a possible prison sentence during the trial so Trump and his loyalists could not accuse them of taking sides against the former president, the official said.

-Josh Margolin