President Obama to address the nation on ISIS tonight at 9 p.m.

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When President Barack Obama outlines his plan Wednesday for combating the ISIS terror group, he'll focus on three major themes, a senior administration official said.

Obama will deliver his speech at 9 p.m. ET.

The President will frame the threat posed by ISIS, outline his strategy to address that threat and share new proposals on how to fight and destroy the militant group, the official told CNN's Jim Acosta on Tuesday.

In the first portion explaining the threat the terror group poses, the official said, the President will offer a big picture of how he views ISIS.

He'll explain that it has become a "core national security priority," and will argue that ISIS cannot be allowed to secure a haven in a part of the world that is borderless.

Obama will lay out his strategy in an effort to counter the perception that he doesn't have a plan to deal with the ISIS threat, according to the official.

This perception was fostered by the President's own words, when he said last month that there was "no strategy" to combat ISIS.

The Wednesday speech will provide Obama the opportunity to specify the actions he has taken, such as building an international coalition that includes Arab partners from the region, and involving Iraqi forces along with potential Syrian rebel combat boots on the ground.

Elements of the President's plans for ISIS are a work in progress, the official cautioned.

While the overall strategy to involve an international coalition will not change, U.S. diplomatic efforts in the works this week could cause some specifics to evolve. Secretary of State John Kerry, Defense Secretary Chuck Hagel and Lisa Monaco, the homeland security adviser, are all traveling to the Middle East this week.

Kerry left Tuesday to push Sunni leaders in Jordan and Saudi Arabia to join the United States and its allies in combating ISIS.

No permission needed from Congress

The official declined to elaborate on whether the President has decided to go ahead with airstrikes in Syria, but administration officials have said the problem must be dealt with on both sides of the border.

The militant group the Islamic State in Iraq and Syria, which calls itself the Islamic State, operates in both nations mentioned in its title.

Obama told congressional leaders Tuesday he has the authority to carry out his planned strategy against ISIS fighters in Iraq and Syria without authorization from legislators.

But show of unity with Congress would help

But Obama has asked for congressional support to show the nation was united.

Obama believes the nation is stronger and its efforts more effective when he works with Congress to battle threats to national security, the White House said in a statement.

"The President told the leaders that he would welcome action by the Congress that would aid the overall effort and demonstrate to the world that the United States is united in defeating the threat," the statement added.

Earlier, two senior legislators -- one from each party -- told CNN's Dana Bash that a congressional vote on military action against the jihadists was unlikely despite calls for one by many of their colleagues.

Volatile issue before election could be risky political move

While some in Congress want to vote on the matter, taking up such a volatile issue as military action weeks before the November elections may be politically dangerous.

Obama has been criticized by conservatives and some Democrats for what they call a timid response so far to the threat by ISIS fighters who emerged from Syria to rampage through northern Iraq.

The President authorized airstrikes on ISIS targets in Iraq this year, but he faces pressure to expand the campaign to go after the extremists in Syria.

Obama and his top aides call such a military step one part of a broader strategy that includes establishing a stable representative government in Iraq, forming an international coalition that includes Middle East countries, and increasing military aid to moderate opposition groups fighting the terror group in Syria.

The top Republicans and Democrats from the House and Senate -- House Speaker John Boehner of Ohio and Senate Republican leader Mitch McConnell of Kentucky, along with Senate Majority Leader Harry Reid of Nevada and House Democratic leader Nancy Pelosi of California -- got the first glimpse of Obama's plan to "degrade and defeat" ISIS at a White House meeting Tuesday.

Boehner said he would support Obama if the President deployed the U.S. military to help target ISIS leadership and train and advise Iraq forces.

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