Nancy Pelosi backs Obama's Iran pact. Is that a big deal?

House minority leader Nancy Pelosi says she’s all-in for President Obama’s Iran deal. On Thursday, the California lawmaker vowed to try and win over skeptical fellow Democrats to prevent the historic agreement from being blocked on Capitol Hill.

The Iran nuclear accord “will have my strong support,” Representative Pelosi said at a news conference.

“I’m very optimistic about our ability to support the president,” Pelosi added.

The Iran deal is far from popular in Congress. Many senators and representatives worry that it allows Tehran to keep too much of its nuclear enrichment infrastructure and that once economic sanctions on Iran are lifted they’ll be impossible to re-impose.

Republicans are likely to oppose the agreement en masse.

“This is a bad deal. We’re going to fight a bad deal,” House Speaker John Boehner (R) of Ohio said Thursday at his own press conference.

Obama argues that the deal prevents Iran from obtaining a nuclear weapon for its duration and is the best agreement the US and its allies can get. Supporters challenge its critics to describe a realistic alternative.

Vice President Joe Biden is taking the lead in lobbying his former Capitol Hill colleagues. On Wednesday, he met with assembled House Democrats for 90 minutes.

“He was spectacular,” Pelosi said of Mr. Biden.

How important is Pelosi’s support in this instance? Very. She could well be the difference between victory and defeat for Obama’s biggest foreign policy initiative.

By law, the administration has to submit the Iran agreement to Congress. Lawmakers will have 60 days to review the pact and vote to approve, disapprove, or remain silent.

It’s quite possible, even likely, that majorities of both the House and Senate will vote to disapprove. President Obama has already promised he’d veto a disapproval motion. That would leave the accord dependent on whether or not the House and Senate can both muster the two-thirds vote necessary to override a presidential veto.

In the House, Pelosi’s support is a necessary condition for Obama to succeed in blocking an override. It’s not sufficient in itself for a presidential victory. But in her long leadership career, Pelosi has proved an effective whip. And blocking a piece of legislation is all that’s at issue. That’s much easier than mustering the greater number of votes necessary for passage.

“Not a surprise, but Pelosi’s backing of the Iran deal is a big deal if it comes down to sustaining a veto,” tweets Bloomberg national political reporter Sahil Kapur.

Remember, Obama only needs inaction to win here. As political scientist Jonathan Bernstein points out in his Bloomberg News column, that means the president only needs to hold Democrats in line for veto sustainment. He doesn’t have the much tougher problem of trying to cobble together a two-thirds majority in favor of the Iran pact, something that would require bipartisan support.

So Obama benefits if the issue becomes politically polarized. That happens when he speaks – there's lots of evidence that presidents tend to polarize public opinion along party lines, points out Mr. Bernstein. That's one reason why Obama held a press conference on the Iran accord on Wednesday.

“Expect more of this,” writes Bernstein.