With Senate Dems increasingly likely to introduce and even vote on a bill imposing new sanctions on Iran, the White House is escalating its behind the scenes pressure on them to hold off, warning them that in moving such a measure, they are making war with Iran more likely.

“Members of Congress pressing for this bill are effectively choosing to close the door on diplomacy, making it far more likely that we’ll be left only with a military option,” one senior administration official tells me, characterizing the message that’s being delivered directly to Senators. “You close the door on diplomacy, and you’re left only with a choice between a possible military option or Iran steadily advancing its nuclear program.”

National Journal reported today that Senator Bob Menendez, the chairman of the Foreign Relations Committee, and other Democrats, along with GOP Senator Mark Kirk, may introduce a bill imposing new sanctions on Iran as early as today. As NJ notes, this would set up the bill to be voted on when the Senate returns in January, and would represent a “bold act of defiance against the administration, which was still begging lawmakers this week to sit back and wait to see whether a comprehensive agreement can be reached.”

The bill Dems may introduce would impose sanctions after the six month deadline in the current, temporary deal, and it would probably have flexibility built in so the White House can delay the sanctions for limited periods, if both sides think a deal is within reach and want to keep talking. Democrats have argued that passing a sanctions bill now would give the White House the flexibility it wants, while also helping the prospects for a longer term deal, by dangling the threat of sanctions later, to increase pressure on Iran.

But the administration has told these Democrats — publicly and privately — that their bill does not give them the flexibility they need and that they don’t need the added pressure. They’ve also said passing a bill now that takes hold in six months would not have a materially different impact than waiting six months before passing one would, even as it could also allow Iran — and the U.S.’s negotiating partners — to argue that the U.S. is not negotiating in good faith.

“It is not necessary for Congress to pass this bill, because we are enforcing existing sanctions and can move to sanctions if negotiations don’t succeed or if Iran cheats,” the senior administration official says. “The fact is, passing new sanctions now would split the international community, embolden Iranian hard-liners, and likely derail any prospect of a diplomatic resolution.”

The push for a new sanctions bill is also splitting Democrats. While Senators like Menendez and Chuck Schumer support such a bill, others oppose this course of action, including Banking Committee chair Tim Johnson, and possibly Harry Reid as well, though he has been quiet. Senators Barbara Boxer and Carl Levin published an op ed today opposing new sanctions, arguing that Congressional action now would “endanger negotiations that most people and countries want to succeed” and could “bolster the efforts of Iran’s militants to kill the deal.”

With some Senate Dems coming out against Congressional action — and with the administration lobbying hard behind the scenes — it’s possible that a sanctions bill could actually go down to defeat in the Senate, which would be a rebuke to the hawks. But it’s very possible one could pass, and if the White House is right, it would put the prospects of a long term diplomatic breakthrough in doubt.