Republican presidential candidate Sen. Lindsey Graham (S.C.) laid out a detailed plan for defeating the Islamic State terrorist group in an interview Wednesday with the Washington Free Beacon, offering a frank assessment of the necessity for more U.S. troops and a two-front strategy for Iraq and Syria.

Concerns continue to be raised about the current U.S. strategy for defeating the Islamic State (IS), which now reportedly controls more than half of Syria and seized the Iraqi city of Ramadi last month. Despite U.S. airstrikes and the presence of about 3,500 troops to train and advise Iraqi forces, the Obama administration’s plan to "degrade and destroy" IS does not appear to have slowed the group’s momentum.

Graham, one of the leading GOP voices on foreign policy and a member of the Senate Armed Services Committee, said "Gen. Obama and [Vice President Joe] Biden have been very lousy generals" in the fight against IS. He added that the withdrawal of all U.S. troops from Iraq in 2011 only emboldened neighboring Iran, which has expanded its influence in Iraq by backing Shiite militias.

"Nothing you see today was unpredictable," he said. "It was the natural, logical consequence of a poor decision by the president."

Graham said he agrees with retired Gen. Jack Keane, one of the architects of the surge strategy in the Iraq War, on the need to deploy about 10,000 U.S. troops to Iraq. This would include aviation battalions and special operations forces that could call in airstrikes and better assist Iraqi forces on the front lines. The raid last month by U.S. commandos that killed a senior IS official in Syria should be replicated, he said.

"If I were president we’d be doing that every night in Syria and Iraq," he said. "They would never get a minute’s sleep."

More U.S. advisers on the ground would also expedite the training of Iraqi forces and Sunni Muslim tribal fighters from the western Anbar province, who remain distrustful of the Shiite-dominated government in Baghdad and its ties to Iran, he said.

Frederick Kagan, director of the Critical Threats Project at the American Enterprise Institute, articulated a similar strategy in congressional testimony on Wednesday. About 20,000 U.S. troops would be required to help call in airstrikes, embed with Iraqi units to improve their effectiveness, conduct raids on IS leaders, and obtain better intelligence, Kagan said.

In a February poll conducted by CNN, 57 percent of Americans said they did not approve of Obama’s efforts to defeat IS. However, a slim majority of respondents said they still opposed sending ground troops back to the Middle East.

As president, Graham said he would tell Americans, "if we don’t deal with the threats over there, they’re coming here."

"I’ve never seen so many terrorist organizations with so many safe havens, so much money, so many men, equipment, and material to hit the homeland as I do today," he said.

IS has also demonstrated an ability to expand its so-called caliphate and global influence, making inroads in Afghanistan, Libya, and Yemen, and inspiring attacks in Paris and Texas.

"This is not a regional conflict," he said. "This is a worldwide struggle of good versus evil, religious Nazis against the rest of us."

Turning to Syria, he said the chaotic civil war there "makes Iraq look like a cakewalk." The Pentagon is attempting to train and equip more moderate Syrian rebels to battle IS, but fewer than 200 fighters have begun the program so far due to vetting issues and the difficulty of exiting the country’s war zone. Graham said the train-and-equip plan would "have to be 150 years old before this thing has any chance of working," noting that IS could have as many as 40,000 fighters in Syria, its original base of operations.

"There are more foreign fighters coming into [IS] ranks than we’re training Free Syrian rebels," he said.

The U.S.-trained rebels also do not have explicit approval to target Syrian dictator Bashar al-Assad, a situation that Graham said has undermined regional support for the effort and must change. He said he would assemble a regional force composed of troops from Egypt, Turkey, and Saudi Arabia to topple Assad and eradicate IS and other terrorist groups in Syria. Yet he added that "big Arab armies haven’t really fought much," meaning that more U.S. commandos will be required to give them a logistical edge and act as forward air controllers to improve the accuracy of airstrikes.

Once IS has been decimated from Iraq and Syria, U.S. allies must hold the territory and begin a process of reconciliation between Sunnis and Shiites, he said. A political settlement could begin to heal a post-Assad Syria, while neutralizing the influence of Iran in Iraq would create more stability in that country and the region. Graham added that he "would walk away" from the current nuclear negotiations between the United States and Iran and ramp up pressure on Tehran with additional sanctions.

Americans must be prepared for the long-term challenge in the region of rooting out IS and rolling back Iran’s influence, he said, dismissing quick solutions such as the partition of Iraq and Syria.

"This mess has got many tentacles to it," he said. "As you pull up the caliphate by its roots, you’ve got to explain to the American people—this is going to be a long struggle to hold this ground, rebuild Syria and Iraq."