After two weeks of vacation spent largely on the golf course in Martha's Vineyard, President Barack Obama heads home to Washington to a full diplomatic plate: Iran, China and a highly anticipated papal visit.

The 44th US president managed to remain relatively low-key during his time off, hitting the beach with his family and publishing his Spotify playlists on Twitter. The White House also made public his summer reading selection.

But when he returns to 1600 Pennsylvania Avenue on Sunday, Obama will hit the ground running on the issue to which he devoted much of his time at the start of the summer -- the controversial Iran nuclear deal.

Of course, the president spent some time at Martha's Vineyard on matters of state, working the phones to rally support in Congress for the agreement, which curbs Tehran's nuclear program in exchange for sanctions relief.

Republican lawmakers -- along with a few maverick Democratic senators -- vehemently oppose the deal, and are likely to pass a resolution of disapproval.

But Obama is certain to veto the measure, and needs only one-third of the members in the House of Representatives and the Senate to back him in order for that veto to stand.

Both sides are lining up their forces -- and courting those who remain on the fence. They are also engaged in a fierce PR battle.

The American Israel Public Affairs Committee (AIPAC), the main pro-Israel lobbying group in the United States, is doing all that it can to keep Obama from carrying out what it calls a "bad deal."

While Obama seems nearly certain of victory, the war of words is being closely watched in Washington -- and Tehran.

"Part of the reason the opposition is spending so much money, even though they know they will likely lose, is because the objective is not just to try to stop the deal," explains Trita Parsi of the pro-deal National Iranian American Council.

"They want to keep the stigmatization of Iran alive as long as possible to ensure that the US and Iran do not move closer to each other even after a successful nuclear deal," he explained.

"If the numbers are very close, the other side will have additional incentives to continue to challenge and weaken the agreement."

Obama, aware that one of his signature foreign policy achievements is at stake, plans to throw all of his weight behind the deal in the coming weeks.

Senate Majority Leader Mitch McConnell admitted Monday that while he hoped Congress could override a veto, the president had "a great likelihood of success."

The country is divided on the issue. According to a CNN/ORC poll released Thursday, 56 percent of Americans say they believe Congress should reject the deal.

And six out of 10 Americans disapprove of the way Obama has handled Washington's relations with the Islamic Republic, the poll revealed.

- Tensions with China -

Another important item on Obama's diplomatic agenda for September is a state visit by Chinese President Xi Jinping, against a backdrop of global fears about the weaknesses of China's economy -- the world's second largest.

Washington and Beijing are at odds on a number of issues -- from China's increasingly aggressive maritime claims, which have angered its Asian neighbors, to its alleged cyber intrusions, including the massive hack of data about US government personnel that many reports have blamed on Beijing.

On September 23, Obama will receive Pope Francis at the White House, with the normalization of US-Cuban relations -- a breakthrough achieved thanks in part to the Vatican -- and the battle on climate change atop the agenda.

The two men are on the same wavelength on both issues, much to the chagrin of skeptical Republicans who think the pontiff went beyond his purview in publishing an encyclical on the environment calling for world action on global warming.

At the end of September, Obama will head to New York for his seventh -- and next-to-last speech before the UN General Assembly, just over a year before he leaves office.

Just ahead of the diplomacy marathon in September, Obama -- who hopes to help seal a global climate deal at talks in Paris in December -- will travel late this month to Alaska, where global warming is of particular concern.

"What's happening in Alaska isn't just a preview of what will happen to the rest of us if we don't take action," he said in a video released by the White House during his vacation.

"It's our wake-up call. The alarm bells are ringing."