"This agreement will substantially constrain the Iranian nuclear program for its duration, and, compared with all realistic alternatives, it is the best option available to us at this time," Casey wrote in a 17-page, more than 8,000-word, analysis. He called it "one of the most difficult decisions of my public career."
In an interview in his Washington office, he added, "I believe that this is better for our security and better for Israel's security, without a doubt, short-term and long-term."
Coons, in a speech at the University of Delaware, expressed deep skepticism of the deal's flaws - "Frankly, this is not the agreement I had hoped for," he said - but added that he saw no credible alternative, and real damage from nixing the agreement.
He said the deal would limit Iran's nuclear program for the next 15 years "with the full support of the international community," while walking away would lead to "uncertainty and likely isolation."
"I support this deal with my eyes wide open, aware of the deal's flaws as well as its potential," said Coons, a member of the Foreign Relations Committee.
He rejected the idea of pushing for a tougher agreement, saying it would divide America from its international partners in the talks - the United Kingdom, France, Germany, China, and Russia - and undermine U.S. authority if it came time to use military force.
The White House needs 34 Senate votes to sustain a potential veto blocking a vote to reject the agreement, and Obama can avoid the spectacle of a veto if he can persuade 41 out of the Senate's 46 Democrats to support him.
New Jersey's Cory Booker - who has strong ties to the Jewish community and Obama - is now the only undecided senator in the Philadelphia region.
Casey, long a hawk on the Middle East and a former member of the Foreign Relations Committee, was seen as a Democrat who might break with the president. But so far only two Senate Democrats - New Jersey's Robert Menendez and New York's Chuck Schumer - have said they will oppose the plan. Sen. Tom Carper (D., Del.) announced his support last week, while the region's only Republican senator, Pennsylvania's Pat Toomey, opposes the agreement.
Backers argue that the deal will prevent Iran from obtaining a nuclear weapon for more than a decade without war, and have pushed Democrats to support one of Obama's most significant foreign policy initiatives. Opponents, including many Jewish groups, warn that the accord will bring Iran closer to obtaining a nuclear weapon and endanger Israel.
The two sides bought nearly $1.3 million of advertising time from Philadelphia broadcast networks in July, August and the early days of September as they targeted a media market that reaches into three states with undecided senators.
Obama hit Philadelphia's airwaves as well, giving an interview to 6ABC last week as he tried to seal support with appearances on several stations around the country.
Casey and Coons each described long, agonizing deliberations.
In a heavily footnoted paper that aides say he largely wrote himself, Casey wrote that he is "skeptical" that Iran will uphold its end of the agreement, and that "one of the most troubling questions" is the idea that billions in sanctions relief for Iran will finance terrorism.
Coons noted that the deal will freeze but not dismantle Iran's nuclear program, and worried that Tehran will continue fueling anti-American and anti-Israeli sentiment.
Both lawmakers urged Obama and future administrations to hold out the use of force as an option if Iran moves to acquire nuclear weapons, and urged the president to strengthen Israel's defenses.
"The Iranian regime should not doubt our capability and willingness to respond swiftly," Casey wrote.
While critics warn that the deal could put Iran in position to obtain a nuclear weapon, both senators noted that Tehran already needs just two to three months to enrich enough uranium for a bomb.
"Your decision on the agreement has to be reactive, or responsive to that reality," Casey said.
The deal would push Iran's time to obtain a nuclear bomb to a year, according to the administration.
It is broadly unpopular, though.
A Quinnipiac University poll released Aug. 24 showed that Pennsylvania voters oppose the agreement 61 percent to 26 percent. Nationally, voters are against it, 55 percent to 25 percent, according to a Quinnipiac survey released Monday.
"The American people are growing more and more opposed to the deal" as they learn more about it, said Mark Wallace, chief executive of one of the leading group's criticizing the deal, United Against Nuclear Iran.
He said the president was twisting Democrats' arms for support and held out hope that votes may change as more information comes out.
The opposition, though, has fallen flat. Casey and Coons added to Obama's support.
"At some point, it has to be a vote, I think, of conscience," Casey said, "and I have many very good friends who will disagree with the decision, I'm certain of that."
Vote-Counting In the Senate
With the support of Sens. Bob Casey and Chris Coons on Tuesday, President Obama is just one vote short of the 34 needed to sustain any veto of a resolution against the nuclear deal with Iran; eight votes would give him the 41 needed to block the resolution from passage.
Among the 46 members of the Democratic caucus, two - New York's Charles E. Schumer and New Jersey's Robert Menendez - are opposed.
Eleven Democrats remain undeclared: Michael Bennet (Colo.), Richard Blumenthal (Conn.), Cory Booker (N.J.), Maria Cantwell (Wash.), Ben Cardin (Md.), Heidi Heitkamp (N.D.), Joe Manchin (W.Va.), Barbara Mikulski (Md.), Gary Peters (Mich.), Mark Warner (Va.), and Ron Wyden (Ore.). - AP