(CNN) -

Republicans in Congress have begun their most forceful push yet in urging President Barack Obama to begin a military campaign into Syria to defeat the Islamic State in Iraq and Syria.

"One of the key decisions the President is going to have to make is air power in Syria. We cannot give (ISIS) a base of operations," Sen. John McCain, R-Arizona, said on "Fox News Sunday."

For nearly three years, Obama has refused to engage in a complicated civil war in Syria where different factions are fighting each other and attempting to topple President Bashar al-Assad.

The President has actively defended his decision to not get involved in the war-torn country. But now that ISIS, which gained prominence and power in Syria, is expanding its scope, the domestic calls for the President to act in Syria are loud.

Conservatives pushing for military action dominated the Sunday political talk shows less than one week after an ISIS militant with a British accent beheaded American journalist James Foley in a video of his killing.

"I don't want to hear from the President about how he's reacting to events," U.S. Rep. Paul Ryan, R-Wisconsin, said on CBS News' "Face the Nation." "I want to hear a strategy how he's going to fend ISIS off."

Sen. Lindsey Graham, R-South Carolina, said on CNN's "State of the Union" that it is to "America's peril" if ISIS is not stopped.

"It's about time now to assume the worst about these guys, rather than to be underestimating them," he said on CNN's "State of the Union."

Defense Secretary Chuck Hagel gave a stark assessment of ISIS on Thursday, indicating the terror group, which has expanded its grip to western Iraq, needs to be addressed.

"This is beyond anything that we've seen. So we must prepare for everything. And the only way you do that is you take a cold, steely, hard look at it and get ready," Hagel said.

At the same news conference, Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, said the U.S. is going to expand its reach to address ISIS. He refused, however, to confirm that the U.S. would conduct airstrikes in Syria.

"It requires a variety of instruments, only one small part of which is airstrikes. I'm not predicting those will occur in Syria, at least not by the United States of America. But it requires the application of all of the tools of national power -- diplomatic, economic, information, military," he told reporters Thursday.

Graham said the United States must do everything it can to defeat ISIS, even if that means U.S. ground troops become engaged.

"If our military commanders tell us that we need ground forces to defeat ISIL, which is a threat to the United States, so be it," he said.

But Sen. Jack Reed of Rhode Island, the only Democrat to appear on the Sunday shows to talk about ISIS, pushed back against the idea that U.S. troops should enter into another ground war.

"The most effective use of our force is not putting troops on the ground, but using capable troops like Kurds with (U.S.) airstrikes," Reed said.

Threat to the United States

The Republican lawmakers said their calls for expanded military action are justified because ISIS poses an "imminent threat" to the United States.

"We should, in my view, look at ISIL as a direct threat to the United States," Graham said, using the alternate name for ISIS.

Graham's assessment mirrors all of the other top Republicans who appeared on political talk shows Sunday, who insisted that ISIS is capable of committing an attack on U.S. soil.

"I do think they present the greatest threat we've seen since 9/11," Rep. Michael McCaul, R-Texas, said on ABC's "This Week."

Top military officials issued dire warnings about the severity of ISIS and their mission without saying ISIS posed an immediate, direct threat to the United States.

Dempsey described their mission as "apocalyptic," and Hagel called the threat "imminent ... to every interest we have."

McCaul, the chairman of the House Homeland Security Committee, said he believes an attack on the U.S. is in the works.

"They would love more than nothing else to hit the United States of America," he added.