President Barack Obama is gradually building support in the U.S. Congress for an international nuclear deal with Iran, working the phones to counter lobbying against the pact and sending a letter to lawmakers urging them to support it.

Obama needs to win the backing of one-third of either the House of Representatives or the Senate to prevent Republicans from killing the nuclear deal announced in July.

Signed by world powers and Iran, the agreement would require Tehran to abide by new limits on its nuclear program in return for western governments easing economic sanctions.

According to a Reuters tally, Obama is eight votes away from capturing one-third of the Senate, or 34 senators, with about a month remaining to find the additional support he needs.

The Bipartisan Policy Center, which is tracking lawmakers’ positions, said on Thursday that 69 House members now support the Iran deal, with another 140 in the 435-member chamber still undeclared. Obama would need the support of at least 146 House members to safeguard the agreement in that chamber.

Even though Congress is on a five-week summer recess and Obama is vacationing in Martha’s Vineyard, the White House has kept up pressure on the president’s fellow Democrats to provide him with enough support when Congress casts votes in September.

Senate Majority Leader Mitch McConnell, a Republican, this month told a Kentucky newspaper that the battle in Congress “is stacked in the president’s favor.”

In a letter to colleagues on Friday, House Democratic leader Nancy Pelosi said she was confident Democrats would prevent an override of a veto by Obama of a measure to kill the agreement.

Obama reiterated his case for the deal in an Aug. 19 letter to legislators that was released by the White House. Reacting to the letter, Democratic Representative Jerrold Nadler of New York on Friday said he would “support the agreement and vote against a resolution of disapproval.”

Representing a New York City district with a large Jewish population, Nadler could help sway other House Democrats.

His move contrasts with other liberal Jewish Democrats from the New York region, such as Senator Charles Schumer and Representatives Elliot Engel and Steve Israel, who have announced their opposition.

In days ahead, much attention will focus on senators Benjamin Cardin and Barbara Mikulski, both senior Democrats from Maryland who have not yet staked out a position.