President Obama meets Tuesday in the Oval Office with, from left, Senate Minority Leader Mitch McConnell (R-Ky.), House Speaker John Boehner (R-Ohio), Senate Majority Leader Harry Reid (D-Nev.) and House Minority Leader Nancy Pelosi (D-Calif.) to discuss the U.S. response to the Islamic State. (Chip Somodevilla/Getty Images)

President Obama is prepared to use U.S. military airstrikes in Syria as part of an expanded campaign to defeat the Islamic State and does not believe he needs formal congressional approval to take that action, according to people who have spoken with the president in recent days.

Obama discussed his plans at a dinner with a bipartisan group of foreign policy experts this week at the White House and made clear his belief that he has the authority to attack the militant Islamist group on both sides of the Iraq-Syria border to protect U.S national security, multiple people who participated in the discussion said. The move to attack in Syria would represent a remarkable escalation in strategy for Obama, who has sought during his presidency to reduce the U.S. military engagement in the Middle East.

Administration officials have been working in recent days to enlist the support of the nation’s political establishment to help sell their strategy to the American public, which Obama will address in a prime-time speech Wednesday night. The president met with the top four congressional leaders Tuesday, while his aides held briefings on Capitol Hill.

The intensity of the outreach amounted to a tacit acknowledgment by administration officials that they have been slow in formulating a strategy to confront the militants and in conveying that vision more broadly. The Islamic State controls wide swaths of territory in both countries, but the United States has so far limited its military engagement to Iraq, as Obama has been reluctant to intervene in Syria’s civil war.

Obama is committed to taking the fight to the Islamic State “wherever their strategic targets are,” said Michèle Flournoy, a former undersecretary of defense for policy who was among those at Monday’s dinner.

“This is not an organization that respects international boundaries,” said Flournoy, who left the Obama administration in 2012 and now serves as chief executive of the Center for a New American Security. “You cannot leave them with a safe haven. . . . I expect him to be very candid.”

There is no indication that a U.S. strike in Syria is imminent, and the Obama administration has signaled that a stepped-up U.S. effort in Iraq, in conjunction with an international and regional partnership, is probably the first step in combating the Islamic State’s advances.

A White House spokeswoman declined to comment for this article, but White House press secretary Josh Earnest said at his daily briefing Tuesday that the Islamic State is “essentially operating in a virtual safe haven in Syria. That’s a dangerous situation.”

Many congressional Republicans, as well as some Democrats, have criticized Obama as being too cautious. House Speaker John A. Boehner (R-Ohio) told reporters Tuesday that “what we need is a strategy,” while Senate Minority Leader Mitch McConnell (R-Ky.) said in a floor speech before meeting with the president that Obama “needs to explain to Congress how additional authorities of the use of force will protect Americans.”

After the meeting with congressional leaders Tuesday, Boehner was supportive of some of what he heard. The speaker’s office issued a statement saying, “The Speaker stated he would support the President if he chose to deploy the military to help train and play an advisory role for the Iraqi Security Forces and assist with lethal targeting of ISIL leadership.” (The Islamic State is also known as ISIL or ISIS.)

Obama informed lawmakers during the session that he did not need new authority from Congress to pursue his approach to countering the threat posed by the Islamic State, according to congressional leadership aides.

Former vice president Richard B. Cheney made his case for more-aggressive intervention in Iraq in a meeting with House Republicans on Tuesday, but many of the newer, more libertarian members of the GOP conference seemed unmoved.

The War Powers Act of 1973 is a classic separation of powers struggle. President Obama has already taken military action to fight the Islamic State in Iraq, but is Syria next? Here’s what the president can do, with or without Congress. (Jackie Kucinich/The Washington Post)

The president is unlikely to lay out all the details of his plans to expand U.S. military engagement in the region during his Wednesday speech, which will take place at 9 p.m. at the White House. Instead, he is expected to state his case for how targeted U.S. military force fits into his broader strategy to develop an international coalition and regional partnerships to attack the Islamic State in order to “degrade and destroy” its operations.

The president “thinks he has the legal authority he needs” to increase U.S. military engagement in both Iraq and Syria, said Jane Harman, president of the Woodrow Wilson International Center for Scholars, who attended the dinner with Obama. The White House’s belief that it has authority to act is based on the reports Obama has filed with Congress under the War Powers Act and the earlier congressional authorization for the war in Iraq.

Over the past month, Obama has authorized airstrikes against the Islamic State in Iraq to defend U.S. personnel, help rescue members of a religious minority group trapped on a mountainside, and defend strategic infrastructure, including two dams. He sent in U.S. troops to help protect the consulate in Irbil, but he has vowed not to send U.S. combat forces in the campaign against the Islamic State, after having withdrawn the remaining U.S. forces from Iraq in late 2011.

Last year, Obama had decided to authorize limited airstrikes against Syrian President Bashar al-Assad for using chemical weapons against rebel fighters, but the president ultimately chose to ask Congress to endorse the move with a formal vote. That effort failed, and the United States did not intervene with direct military action. But the Islamic State’s rapid advances into Iraq this summer and its brutal tactics, which have included the beheadings of two U.S. journalists in the past month, have ramped up pressure on the administration to take stronger action.

A Washington Post-ABC News poll this week showed that Americans overwhelmingly view the Islamic State as a serious threat to vital U.S. interests and, in a significant shift, widely support airstrikes in Iraq and Syria. Seventy-one percent of all Americans say they support airstrikes in Iraq, and 65 percent support strikes in Syria. That is more than double the level of support a year ago for launching airstrikes to punish the Syrian regime.

Several prominent Republicans, including Sen. Robert Portman (Ohio), said they believe that Obama can launch airstrikes to combat the militants in both Iraq and Syria without seeking a congressional vote.

“The president has authorization to act now,” Portman said in a phone interview Tuesday, adding that he remained “a little confused” about an administration strategy that has been slow to develop.

“We are ‘leading from behind,’ to use the president’s words,” he said. “By not leading, we are making it more difficult and more costly.”

Obama’s dinner with the foreign policy experts — a gathering that included former high-ranking officials in Republican administrations, including Richard Haass and Stephen Hadley — was part of an intensive White House effort in recent months to expand its sphere of influence. Although the president has on a semi-regular basis invited in outside experts for conversations, the White House has shifted its tone amid a series of high-profile foreign policy crises, including Russia’s intervention in eastern Ukraine, the Israeli-Palestinian conflict in Gaza and the advance of the Islamic State deep into Iraq.

In private conversations, senior White House national security aides have expressed greater willingness to hear new ideas and solicit input in hopes of doing a better job of developing strategy and communicating it to the public, said several influential foreign policy experts who have talked with Obama advisers in recent months.

Jon B. Alterman, who directs the Middle East program at the Center for Strategic and International Studies, said the real challenge Obama faces is not articulating his approach but executing it and brokering the political deals that will underpin any military strike.

“The problem is policy follow-through,” Alterman said, adding that the danger of leading with “a military operation is you start thinking the job is done, when the job has just begun. What the U.S. strategy is trying to do is deny haven to [the Islamic State], and doing it in Iraq with a government that is chaotic at best will take some time. Doing it in Syria is going to be even harder.”

Samuel R. “Sandy” Berger, who served as President Bill Clinton’s national security adviser and attended Monday’s dinner, said Obama “has his arms around this” and is prepared to “lay out a comprehensive strategy to deal with ISIS long-term.”

Harman described the dinner Monday as “focused and thoughtful.” Over a dinner of D’Anjou pear salad and Chilean sea bass, Obama, Vice President Biden and the outside experts engaged in a deep discussion of the options to combat the Islamic State, those who participated said.

Obama was clear that he wanted general congressional support — or “buy-in” — for his plans, in part because the increased military campaign, including training of Iraqi military forces and other regional fighters to take on the Islamic State, will come with a significant price tag from the Pentagon and is likely to take several years, people involved in the dinner said.

One former Obama administration official, who was not at the dinner Monday and spoke on the condition of anonymity to discuss private conversations with the administration, said the president was committed to a strategy to strike the Islamic State “wherever they are.” The ex-official added: “The bar for strikes in eastern Syria is higher than in western Iraq, and if you’re a military planner you have to meet that threshold. But the fact that we’re reportedly doing surveillance flights over Syria already — you’re not doing those things unless the military has already been asked to prepare a targets package.”

Katie Zezima, Ed O’Keefe and Robert Costa contributed to this report.