Sen. Chris Coons (D-Del.) questions the Internal Revenue Service commissioner during a July 29 hearing on Capitol Hill. In recent months, Coons has emerged as one of the most studious examiners of the Iran nuclear deal. (Chip Somodevilla/Getty Images)

Before he served as a U.S. senator, before he ran Delaware’s largest county, Christopher A. Coons was a corporate lawyer, reviewing contracts and business proposals for W.L. Gore & Associates.

The eye he once brought to sales agreements for Gore-Tex fabric he is bringing — a decade later — to the United States’ most scrutinized diplomatic accord in a generation: the Iran nuclear deal.

“Your training and your role as a lawyer is not to be the wedding-day guy but to be the divorce-day guy,” Coons, a Democratic member of the Senate Foreign Relations Committee, said this past month. “Your job is to read it through as closely as you can . . . because no one ever pulls that document out unless there’s a problem.”

His lawyerly conclusion: “I will tell you, my gut response at several places has been grave concern.”

Congress is not scheduled to weigh in on the Iran agreement for at least six more weeks, but its fate lies in the hands of undecided lawmakers such as Coons who have assumed a meticulous, rigorous approach to making a decision.

And, uncomfortably for President Obama and his negotiating team, nearly all of them are Democrats.

Their reviews of the accord could leave the outcome of a congressional vote uncertain for weeks, spanning the summer congressional recess when advocates on both sides are expected to pressure the undecided.

Most notably, the American ­Israel Public Affairs Committee last month formed a new anti-deal lobbying group whose board is packed with former Democratic lawmakers, including former vice presidential nominee and senator Joseph I. Lieberman (I-Conn.). Backers of that group, Citizens for a Nuclear-Free Iran, have pledged to raise as much as $40 million. Meanwhile, Israeli Ambassador Ron Dermer has met with more than 50 members of Congress to warn against the deal, with more meetings planned for this week, an Israeli official said.

On the other side, the liberal pro-Israel group J Street has said it plans to raise millions of dollars to support the deal, and the White House has promised to remain engaged throughout the two-month review. Obama is expected to set the tone with an address Wednesday at American University in Washington, the site of President John F. Kennedy’s 1963 speech calling for high-stakes nuclear negotiations with the Soviet Union during the height of Cold War.

The White House also held a lobbying session for dozens of lawmakers this past week, and Obama has made phone calls to skeptical but undecided Democrats.

“You think that happens every week?” said Rep. Brad Sherman (Calif.), who took one such call from the president.

In the House, 150 Democrats signed a letter last month supporting the concept of an Iran deal. If all were to vote in favor of the accord, it would be enough to sustain an Obama veto. But opponents think they will be able to pick off supporters as they publicize what they describe as weaknesses in the agreement — in particular, the lack of “anywhere, anytime” inspections, the prospect of lifting a conventional arms embargo, and uncertainty about private agreements between international nuclear arms monitors and the Iranian regime.

In the Senate, as in the House, there is little doubt that a majority opposes the deal. Of 54 Republican senators, only Jeff Flake (Ariz.) has signaled that he might support it.

The more important calculation centers on whether at least 60 senators will oppose the accord. If not, it would spare Obama the necessity of a veto and the drama of a subsequent override vote.

One Democratic senator, Robert Menendez (N.J.), has been critical of the negotiations and is expected to oppose the deal. A handful of other Democrats also have been critical, if less sharply so, and as many as a dozen senators have taken pains to remain undecided.

They include Michael F. Bennet (Colo.), whose 2016 reelection bid is being heavily targeted by Republicans, and centrists such as Joe Donnelly (Ind.), Heidi Heitkamp (N.D.), Joe Manchin III (W.Va.), Bill Nelson (Fla.) and Mark R. Warner (Va.). And perhaps no decision is being watched more closely than that of Charles E. Schumer (N.Y.), who is caught between his role as the Senate’s Democratic leader in waiting and his long-standing alliance with pro-Israel groups. He has responded tersely in recent weeks to inquiries about his position.

Said Sen. Benjamin L. Cardin (Md.), the ranking Democrat on the Senate Foreign Relations Committee, who remains undecided: “Most Democrats have taken this review process very seriously. They know it’s a complicated situation. It’s not a yes-or-no judgment that should be made without knowing the facts.”

When Secretary of State John F. Kerry, Treasury Secretary Jack Lew and Energy Secretary Ernest Moniz defended the deal before the Senate Foreign Relations Committee this past month, panel members packed the dais. Occupying a front-row seat was Sen. Angus King (I-Maine), who does not sit on the committee but whose vote will count.

“This is maybe the most important vote I’ll ever take in this body,” he said.

King, a former lawyer like Coons and 51 other senators, said his first read-through of the agreement prompted him to scratch 39 questions into the margins — which were sent to aides for answers. King, a member of the Senate Intelligence Committee, also has asked for classified briefings from national security officials.

One step that neither King nor most other undecided Democratic senators have taken is to schedule town-hall-style meetings to hear from constituents.

Manchin is one of the few not bowing to the unspoken fear that such events could be political minefields, much as health-care-themed town hall meetings became in the summer of 2009.

“The facts that are presented to me, that I make decisions on, I want the people of West Virginia to see,” he said last week. “I’m going to editorial boards, I’m going to go do town hall meetings. I’ll do it all.”

Several other Democrats said that they are regularly in touch with their constituents in other ways and that they see no need to hold dedicated events. Cardin said that public input is “part of the process, but it’s not a popularity issue.”

Said King: “People don’t have any trouble offering their opinion in coffee shops or on the street or anywhere else. I’m a walking focus group.”

Coons said the constituent input he has received through phone calls, e-mails and personal appeals has been almost equally divided. He has been more focused, he said, on gathering as much information about the deal as possible — classified briefings from technical experts, talks with foreign negotiating partners and dozens of conversations with administration officials — including Vice President Biden, whose former Senate seat Coons holds.

Coons’s serious approach to reviewing the deal has won him the attention and respect of advocates on both sides of the issue. Many lawmakers are just now taking “Iran 101,” one senior Israeli official said, but Coons is “in the Iran Advanced Placement 707 seminar.”

That is not to say politics is far from Coons’s mind. But with his next election five years away, he is more concerned about the possibility that — as with the ill-fated 1994 North Korean nuclear agreement — the real fallout might not be apparent for years to come.

“There’s a reasonable chance that I may be here long enough,” he said, “that the longer-term outcome of this is a real consequence.”

Paul Kane contributed to this report.