President Barack Obama is promising Democratic lawmakers that the U.S. will continue to keep economic pressure on Iran — and keep military options open — if his administration's nuclear deal with Tehran goes through, the Associated Press reports.

Obama, in a letter addressed to New York Democratic Rep. Jerrold Nadler, said that if Iran rushes to build a nuclear weapon, "all of the options available to the United States — including the military option — will remain available."

The President also says the U.S. will uphold sanctions targeting Iran's non-nuclear activities, such as its support for Lebanon's Hezbollah group and what Obama calls Iran's "destabilizing role in Yemen."

Obama wrote the letter, dated Aug 19, from Martha's Vineyard, the tony island off the Massachusetts coast where he is in the midst of a two week vacation. While the president has made no public appearances during his vacation, he has been privately reaching out to Democratic lawmakers in a bid to boost support for the Iran deal, according to the AP.

Congress will vote next month on a resolution of disapproval on the accord to curb Iran's nuclear program in exchange for billions of dollars in sanctions relief. The President is expected to have to veto the measure, but White House officials and Democratic lawmakers have expressed confidence that there is enough support in the party to block GOP override efforts.

Obama has picked up crucial support from Democrats in recent days, including Sen. Claire McCaskill, D-Mo., who said that while the agreement isn't perfect, it was clear to her that "the world is united behind this agreement with the exception of the government of Israel."

Nadler, who represents a heavily Jewish congressional district, is among the undecided Democrats. Sen. Chuck Schumer, another influential Jewish lawmaker from New York, announced earlier in the month that he would vote against the agreement.

In his letter to Nadler, Obama emphasized U.S. support for Israel, saying he views the country's security as sacrosanct. He said he was committed to deepening missile defense funding and other military cooperation with Israel.