WASHINGTON (AFP) - US President Barack Obama will hold a press conference on Wednesday as he attempts to convince Americans, allies and skeptical lawmakers about the benefits of a nuclear deal with Iran.

Obama will answer questions in the East Room of the White House, spokesman Josh Earnest said on Tuesday.

With Congress set to vote on the deal and would-be Republicans clamouring to denounce it, the White House is aggressively making its case.

Obama must garner the backing of a majority of Democrats in the Republican-controlled Congress to keep the deal alive.

Earnest said Obama had already been briefing some lawmakers on the deal.

The presumptive favourite to be the Democratic nominee, Hillary Clinton, was also contacted.

Behind the scenes, the White House has been briefing Congress, journalists, foreign governments and non-governmental groups about the talks since a framework agreement was reached in Switzerland in April.

But news of Tuesday's deal has sparked a fierce lobbying battle in Washington.

Experts have come out "for" and "against" as both sides try to sway public opinion.

Lobby groups have deployment multi-million-dollar budgets to make their case.

Obama's supporters fear a tight vote in Congress, even one in which the president prevails, could open the door for sustained attack on the deal.

Many Republican candidates to be president in 2016 have already vowed to repeal it.

Amid this barrage, the White House is expected to launch a communications campaign comparable to anything Obama's administration has done to date.

Cabinet members, senior officials and the president himself have already rolled out to sell the deal.