WASHINGTON — Until now, the Obama administration has insisted that the US military response to the Islamic State, primarily in the form of airstrikes in northern Iraq, would be limited “in nature, duration, and scope” and not involve ground combat.

But as President Obama prepares to address the nation at 9 p.m. Wednesday to unveil his overall strategy — possibly including expanding airstrikes into Syria — he will have to convince a wary public and Congress that victory can be accomplished without re-inserting significant numbers of American ground troops.

While there appears to be support across the political spectrum to use military force to confront the group — from Senators Bernie Sanders, the Vermont independent, to John McCain, the Arizona Republican — there remains deep concern that the United States will get drawn into another open-ended ground war in the Middle East.


“People say we are not putting boots on the ground,” said Representative James McGovern, a Worcester Democrat. “We do have boots on the ground. We have gotten more deeply involved.”

Over the past month, as US aircraft have launched more than 150 strikes against the brutal group, the number of US military advisers and security personnel dispatched to Iraq has also grown to nearly 1,000.

Throughout the crisis, Obama has maintained that he has the power to act alone and does not need congressional approval to take military action, writing to Congress on Monday that his actions “are pursuant to my constitutional authority to conduct US foreign relations and as commander-in-chief.”

But he is facing growing demands from members of both parties to seek a debate about the threat and a formal vote on the scope of the American response.

The Democratic chairman of the Senate Foreign Relations Committee, Robert Menendez of New Jersey, has said Obama has “no choice” but to seek congressional authorization.


Similar concerns are likely in the House. In July, by a vote of 370 to 40, a nonbinding resolution cosponsored by McGovern was approved that prohibits Obama from “deploying or maintaining US armed forces in a sustained combat role in Iraq” without a formal congressional vote.

Representative Adam Schiff of California, the senior Democrat on the Intelligence Committee, said in a statement Tuesday he believes the threat posed by the group “is very real and must be confronted.”

“However, if the president intends to prolong the military campaign in Iraq or extend it into Syria, he needs to make the case directly to the American people and secure authorization from Congress.”

Laying the groundwork for his speech to the public, Obama met with congressional leaders at the White House Tuesday.

Some experts assert that a successful campaign might require not just significantly more air power but more US troops on the ground — especially if the Iraqi government and other allies are not up to the task of taking the fight to the militants.

“The question is — and will continue to be — what is the mix that we need in terms of forces?” said Walter D. Givhan, a retired Air Force major general and fighter pilot. “How much do you need on the ground? Or can it be a small footprint, aiding and advising the Iraqi forces as they bring pressure to bear and helping them with the targeting of air power?’’


Givhan, who oversaw the building of the new Afghan air force and is now a senior vice chancellor at Troy University, said that US air power “may not be a comprehensive and complete solution to the problem.”

A leading national security think tank, the Center for a New American Security, also urged Obama on Tuesday not to mislead the public on what might be required, saying he should assert that while “extensive deployments of uniformed combat personnel is not something that he is considering,” more US military personnel on the ground may be required.

Others said it is uncertain whether sustained ground combat will be required to accomplish the administration’s goal.

“Can we win in Iraq and Syria without US ground troops? No one knows for sure,” said Wesley K. Clark, a former Army general who oversaw the NATO air war in Kosovo in 1999. “If you have special forces, good intelligence collection, and responsive allies, I think you can do a lot.”

Clark said he expects the US military involvement to be enduring. “This is not three months and out,” said Clark, a distinguished fellow at the Burkle Center at the University of California, Los Angeles. “This is a long-term strategy of engagement.”

It is for that reason that Obama will be under pressure to seek congressional approval in the coming days for his new military strategy.

Senator Bob Corker, Republican of Tennessee, said he believes “it’s lacking in judgment not to come to Congress on anything that could last this long.”


McCain, a leading hawk who believes US troops should never had been withdrawn from Iraq in 2011, said, “I would love to see a vote. We should be debating right now what to do about” the Islamic State.

“This is not the king’s army,” said Representative Scott Rigell, a Virginia Republican whose district has a large military population.

He said he believes the public has become “desensitized” to the use of force to the dangerous point that sending Americans to war has become almost “theoretical.”

“People want to know what the hell we are getting into,” McGovern said. “To basically say we shouldn’t have a debate because we don’t know how the vote will turn out or because it’s politically inconvenient. . . that’s shrinking from our responsibilities.”

Bryan Bender can be reached at bryan.bender@globe.com. Jessica Meyers can be reached at jessica.meyers@globe.com.