Former President Donald Trump has been making calls to Congressional allies on Capitol Hill Thursday night, urging them to go on the offensive and defend him following the news of the indictment by a Manhattan grand jury, sources familiar with the matter tell ABC News.

Trump has been asking members who support him to firm up their support and rally behind him, the sources said.

-ABC News' Rachel Scott, Will Steakin and Katherine Faulders

Some of Trump's confirmed or prospective rivals for the 2024 Republican presidential nomination were among those who spoke out Thursday night in the wake of the news of the former president's indictment.

In an interview with CNN on Thursday night, former Vice President Mike Pence called it an "outrage," arguing that the case against Trump is "tenuous" and will "only further serve to divide our country."

Florida Gov. Ron DeSantis, whom sources have told ABC News is expected to launch a presidential campaign in the coming months, tweeted that the indictment was "un-American" and "a weaponization of the legal system," adding that Florida would "not assist in an extradition request."

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-ABC News' Alexandra Hutzler and Mariam Khan

California Rep. Adam Schiff said "it's a sober moment for the country," telling Linsey Davis on ABC News Live on Thursday that the dignity of the Oval Office "will be tarnished by the conduct of the former president by his being charged criminally."

"I think you have to be guided by the facts and the law, and you have to set aside the political calendar and do what the law requires," said Schiff, a leading Democrat in the House. "I think that's the obligation of a district attorney, and I think that was done here. How this cuts politically, I really don't know. That, to me, is very secondary."

-ABC News' Imtiyaz Delawala and Anna Katharine Ping

Demonstrators gathered outside Manhattan Criminal Court and near Donald Trump's Mar-a-Lago estate in Florida in the wake of the indictment Thursday evening.

In Manhattan, a giant sign that stated "Trump lies all the time" could be seen unfurled outside Manhattan Criminal Court, where police had erected barricades last week ahead of a possible indictment.

Meanwhile, several supporters gathered near Mar-a-Lago with Trump 2024 flags and signs.

Former President Donald Trump can still be elected president -- even if he is convicted -- experts tell ABC News. But there are practical reasons that could make it a challenge, experts say.

Trump said recently at the annual Conservative Political Action Conference that he would "absolutely" stay in the race for president even if he were to be criminally indicted.

Trump has denied wrongdoing and has characterized the probe as part of a "witch hunt" against him.

The U.S. Constitution does not list the absence of a criminal record as a qualification for the presidency.

Constitutional experts also told ABC News that previous Supreme Court rulings hold that Congress cannot add qualifications to the office of the president. In addition, a state cannot prohibit indicted or convicted felons from running for federal office.

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-ABC News' Laura Romero