U.S. President Barack Obama will shortly be announcing a series of new executive actions aimed at reducing gun violence.

But even before he could open his mouth to explain his plans, critics—mostly from the Republican camp, particularly the GOP presidential candidates—are already calling his initiatives as unconstitutional and as part of his efforts to disarm America.

White House officials said Obama would announce the package of gun control measures on Tuesday, the Washington Post reported.

One key provision, according to the Post, would require more gun sellers to be licensed and would force them to conduct background checks on potential buyers.

Obama appears to be gearing up for a fight against his detractors. In a Twitter message on Monday evening, the president said, "The gun lobby may be holding Congress hostage, but they can't hold America hostage. We can't accept this carnage in our communities," referring to the National Rifle Association (NRA).

Republicans in Congress and on the campaign trail immediately blasted Obama's forthcoming executive actions. Some gun rights advocates threatened to challenge them in court.

House Speaker Paul D. Ryan (R-Wisconsin) issued a statement Monday saying that even without knowing the plan's details, he thinks "the president is at minimum subverting the legislative branch, and potentially overturning its will."

"This is a dangerous level of executive overreach, and the country will not stand for it," Ryan said.

At least four Republican presidential candidates have already assailed the president's plans.

Sen. Ted Cruz (R-Texas) called Obama's move as "illegal and unconstitutional."

Sen. Marco Rubio (R-Florida), for his part, said Obama "has waged war on the Constitution."

"You can pass all the gun laws in the world that you want," he said. "It will not stop the criminals," Rubio added.

"The president is a petulant child," New Jersey Gov. Chris Christie told "Fox News Sunday." "Whenever he doesn't get what he wants...this president acts like a king."

Carly Fiorina told CNN Obama is acting like "a lawless president."

"It is delusional, dangerous, not to mention unconstitutional," she said. "We have a long list of criminals who own guns, who routinely purchase guns. We know who these people are, and we are not prosecuting any of them."

In a tweet, Texas Governor Greg Abbott issued a direct challenge: "Obama wants to impose more gun control. My response? COME & TAKE IT."

However, Democratic presidential front-runner Hillary Clinton hailed Obama's plan during a campaign stop in Iowa on Monday, saying she would go even further as president.

Fox News columnist Todd Starnes said if Obama really wants to crack down on gun violence, all that needs to be done is to strictly enforce the laws that are already in place.

"But that's not the point. This president ultimately wants to disarm the nation," Starnes said.

He said the principal reason America's Founding Fathers wrote the Second Amendment "was to protect all the other amendments."

Starnes noted that after the Muslim terrorist attack in San Bernardino the Washington Post found that 53 percent of voters oppose a ban on assault weapons—a record high.

"The American people seem to understand what the president does not—guns keep our families safe," Starnes said.

"So instead of declaring war on law-abiding gun owners—maybe the president ought to declare war on the true threat facing our nation—radical Islam," he added.