By Donovan Slack

Gannett Washington Bureau

WASHINGTON -- Wisconsin Sen. Ron Johnson has been a frequent critic of President Barack Obama, excoriating his deficit spending, suing him over Obamacare and lashing out at his strategy to take on Islamic State terrorists.

But this week, Johnson is playing a new role: Obama cheerleader-in-chief.

As one of the president's appointees to represent the United States at the United Nations General Assembly, Johnson has been with Obama and State Department officials in New York and is supporting their efforts to build a broad, international coalition to fight the Islamic State in Iraq and Syria.

He has met with officials from Turkey, Egypt and Jordan and asked them to sign on long term, and has plans to meet with Saudi officials as well. Johnson had high praise for Obama's address to the U.N. and his assertion that the ISIS "network of death" must be eliminated.

"I appreciate the fact that President Obama is facing this reality and utilizing the General Assembly to make that point, to raise America's -- and quite honestly, the world's -- awareness of how serious a threat ISIS really is, and extreme Islamic ideology is," Johnson said in an interview Thursday.

The Oshkosh Republican said he spoke with the president by phone for about 20 minutes last week, after the White House announced his appointment as U.N. representative, and Johnson said he told Obama that he wanted to support him in his efforts.

Johnson asked the president whether State Department officials would help him get up to speed on Obama's strategy for this week and set up substantive meetings where he could help push that strategy forward.

"You've always heard the basic approach that politics should end at the water's edge. I think most members of the (Senate) Foreign Relations Committee believe that," said Johnson, who is a member of the committee. "We try and embrace that type of approach, and that's certainly what I want to do here."

It was a sharp reversal from just two weeks ago, when Johnson lambasted Obama on foreign policy, saying his strategy to fight ISIS was weak. He likened it to poking a hornet's nest with a stick rather than eliminating it. He said at the time that Obama never should have announced his plans not to commit any ground troops to the conflict.

Johnson said this week that whatever his earlier criticism, the ISIS threat demands a unified response.

"I want to be supportive of this president because he has to succeed, his administration has to succeed," he said. "If he does succeed, that means America and Americans stay safe. So I want to do everything I can to support his actions."

He attended Obama's address on Tuesday and sat in on the U.N. Security Council meeting chaired by the president on Wednesday. The council unanimously passed a resolution requiring countries to take actions to stem the flow of foreign jihadists to Iraq and Syria. Johnson was struck by the magnitude of the occasion. The council has convened such a meeting with heads of state only six times in its history.

"Only twice has an American president chaired that meeting," he said. "Again, that's one of the ways an American president can highlight how serious this issue is."

Johnson said he tried to buttress that point in his meetings with officials from countries in the Middle East. He said he was receptive to their concerns that public opinion within their countries may not support their partnering with the United States.

"There are legitimate concerns that these regimes have in terms of their own public opinion and so we need to be sensitive to that, but they need to understand that we need to be sensitive to our public opinion as well," he said.

"And there's no way that the American public is going to support the long-term type of action to really preserve the peace in that region if we don't see that the individuals, the states, the regimes in the region are every bitif not more committed to this fight than we are."

Johnson was scheduled to participate in another full day of meetings on Thursday before returning to Wisconsin on Friday.

Contact dslack@usatoday.com. Follow @donovanslack.

