By design or accident, it is increasingly clear that the centerpiece of President Barack Obama’s second-term foreign policy is a nuclear agreement with Iran. Whether Obama can succeed, however, now depends on Congress staying out of the negotiations.

Over the last few weeks, 16 Democratic senators have supported a bill that would impose new sanctions on Iran over its nuclear program. They have defied the White House’s intense campaign to block Congress from adding new conditions to any deal.

In this way, Obama is the victim of an increasingly craven Washington — where members of his own party are abandoning him out of political expedience. At the same time, the White House is also a victim of its sometimes erratic responses to events in the Middle East.

For the last six years, the president has repeatedly declared that he does not want the United States entangled in another conflict in the Middle East. As a result, allies and enemies at home and abroad, from members of Congress to Israeli and Iranian hawks, question his commitment to use force against Iran if negotiations fail.

Experts warn that the stakes are enormous. Political opportunism, maximalist positions and mixed messages could take on a life of their own, scuttle the talks and inadvertently spark military action.

George Perkovich, director of the Nuclear Policy Program at the Carnegie Endowment for International Peace, lambasted the bill’s congressional sponsors in Foreign Affairs. He accused Senators Robert Menendez (D-N.J.), Charles Schumer (D-N.Y.) and Mark Kirk (R-Ill.) of reckless grandstanding.

“The Menendez-Kirk-Schumer bill may be politically expedient,” Perkovich wrote, “but it is also entirely unnecessary and dangerous.”

Much of the Democrats’ maneuvering is old-fashioned political posturing. All the Democratic officeholders now supporting the sanctions bill, David Weigel noted in Slate Tuesday, face tough re-election battles. Rejecting calls from the American Israel Public Affairs Committee to support the new sanctions bill could make them vulnerable to attacks of capitulating to Iran. So far, Democrats from “safer, bluer” turf — including Senators Tim Kaine (D-Va.) and Chris Murphy (D-Conn.) — are not supporting the bill.

Ambition also plays a role here. Schumer, who is safe in New York, is looking to succeed Senator Harry Reid (D-Nev.) as majority leader. His chief rival for this job, Senator Dick Durban (D-Ill.), who was the senior senator from Illinois when Obama was the junior senator, is backing the administration.

Democrats who support the new sanctions bill claim that their goal is to give Obama greater leverage in talks with Tehran. But Perkovich and other experts warn that the proposed sanctions threaten to spark a tit-for-tat cycle of escalation.

As American hard-liners saber rattle, Iranian hard-liners are saber rattling back. If Congress does pass the new sanctions bill, a senior member of the Iranian parliament has threatened, his nation would respond by beginning to enrich uranium to 60 percent — a level close to that needed for a nuclear bomb.

The major unresolved issue — and the biggest threat to a comprehensive deal — is whether Iran should be allowed any enrichment capability.

The White House has signaled that it would accept a tightly monitored program in Iran — one that enriches uranium only to the level used for energy and research.

Israeli Prime Minister Benjamin Netanyahu and hawkish members of Congress argue that increased sanctions will force the regime to give up enrichment or collapse.

Reza Marashi, research director of the National Iranian American Council, an advocacy group that supports the nuclear talks, said it is political suicide for any Iranian official to accept no enrichment. Tehran’s hard-liners would accuse them of capitulation to the United States and Israel.

“I don’t know any Iran analyst — except for those on the far, far right,” Marashi told me in a telephone interview Tuesday, “who think that zero enrichment is possible.”

Obama has also made foreign policy missteps. As I wrote last week the administration’s shifting positions on Syria — from demanding President Bashar al-Assad “must go” to declaring “red lines” on chemical weapons use and then backing away from military action — has hurt his credibility in the region.

Perkovich said domestic missteps have played a role as well. The interim agreement with Iran was announced just as the Obamacare website began its botched rollout. Congressional Democrats facing tough re-election battles decided they simply could not trust the White House.

“The timing was disastrous [to Congress],” Perkovich told me in a telephone interview Tuesday. “They thought ‘these guys are totally incompetent.’”

In addition, the president’s disinterest — or inability — to develop close relationships with members of Congress is now coming back to haunt him. As former Defense Secretary Bill Gates noted in his new memoir, Duty, Obama and President George W. Bush each loathed dealing with Congress.

“Both, I believe, detested Congress,” Gates writes, “and resented having to deal with it, including members of their own party.”

Perkovich argues that Congress should allow negotiations to succeed or fail. A deal that blocks Iran from obtaining a weapon would bolster the nuclear non-proliferation regime, in place since the 1970s, and reduce tensions in the Middle East. A collapse in the talks would weaken the non-proliferation regime and even spark a U.S.-Israeli military strike on Iran.

More decisive leadership from Obama and less opportunism from Democratic senators will not magically stabilize the Middle East. But there is no need for Democratic senators to add to the chaos for political gain.

PHOTO (TOP): President Barack Obama speaks during his year-end news conference in the White House briefing room in Washington, December 20, 2013. REUTERS/Larry Downing

PHOTO (INSERT 1): Iran’s Foreign Minister Mohammad Javad Zarif speaks during a news conference following the EU+3-Iran talks in Geneva, November 24, 2013. REUTERS/Ruben Sprich

PHOTO (INSERT 2): President Barack Obama and Israel’s Prime Minister Benjamin Netanyahu shake hands at a joint news conference in Jerusalem, March 20, 2013. REUTERS/Larry Downing