The warning from Democrats to Republican leaders about undermining President Obama's Iran diplomacy has split party leaders.

One hundred and fifty House Democrats wrote a letter to Obama Thursday urging him to exhaust all diplomatic avenues to a nuclear deal before pursuing tougher options like sanctions � a strong indication that Democrats are ready to block any Republican effort rejecting such an agreement.

But while top leaders like Reps. Nancy Pelosi (D-Calif.), Jim Clyburn (D-S.C.) and Xavier Becerra (Calif.) endorsed the May 7 letter, Reps. Steny Hoyer (Md.), the minority whip, and Joe Crowley (N.Y.), vice chairman of the Democratic Caucus, did not.
The same divide does not surround legislation that would give Congress an up-or-down vote on any nuclear deal finalized between Iran, the United States and five other global powers. Democratic leaders have endorsed that legislation, which sailed through the Senate 98-1 on Thursday and is expected to soon pass through the House with broad bipartisan support.

But the Democrats' letter is widely viewed as a warning shot to Republicans that they have the numbers to sustain a presidential veto if the GOP uses the power provided by the Senate bill to disapprove of an Iran deal down the road.

Some liberal groups, which have lobbied hard for the diplomatic approach, are accusing the Democratic holdouts of siding with defense hawks �who want to rush to war,� in the words of Nick Berning, spokesman for MoveOn.

Both Hoyer and Crowley say they're pushing for a diplomatic solution, but their decision to forego the letter highlights the prickly politics facing lawmakers as they grapple with opposition from Israeli leaders, concerns from Israel's top lobbyist group and near-universal distrust of the Iranian regime.

Hoyer on Friday reiterated his support for peaceful negotiations without answering the question of why he declined to endorse the letter.

�As I�ve said before, a negotiated, diplomatic agreement is the preferred outcome, and Congress has an important role to play in reviewing any agreement,� he said in an email.

Crowley issued a similarly brief statement.

"These current diplomatic negotiations deserve the chance to succeed in preventing Iran from obtaining a nuclear weapon, and I look forward to carefully reviewing a final deal,� he said.

Dylan Williams, vice president of government affairs at J Street, which supports Obama's diplomatic approach and is praising the Democrats' letter, said he's not concerned that some top Democrats withheld their signatures. The fact that 150 lawmakers are on the record backing the framework agreement, he said Friday, "is an indication that support for such a deal likely goes far beyond just the signers of the letter."

Spearheaded by Reps. Jan Schakowsky (Ill.), Lloyd Doggett (Texas) and David Price (N.C.), the Democrats' May 7 letter urges the administration to "stay the course" and "exhaust every avenue" in its bid to craft a diplomatic deal to prevent a nuclear-armed Iran.

"If the United States were to abandon negotiations or cause their collapse, not only would we fail to peacefully prevent a nuclear armed Iran, we would make that outcome more likely," the Democrats wrote.

"We must allow our negotiating team the space and time necessary to build on the progress made in the political framework and turn it into a long-term, verifiable agreement."

Republicans see the framework differently, warning that the terms outlined thus far don't go far enough to block Iran's nuclear weapons ambitions. Negotiators have set a June 30 deadline for finalizing the details.

The Senate legislation, sponsored by Sens. Bob Corker (R-Tenn.) and Ben Cardin (D-Md.), was designed to give Capitol Hill lawmakers a greater voice in the diplomatic effort. The legislation empowers Congress with an up-or-down vote on any deal the administration reaches.

Speaker John Boehner (R-Ohio) praised the legislation after its passage through the Senate on Thursday, saying it's a necessary step for holding the White House accountable to voters.

"Our goal is to stop a bad agreement that could pave the way to a nuclear-armed Iran, set off a regional nuclear arms race, and strengthen and legitimize the government of Iran � which threatens Israel and other allies in the region, as well as supports terrorism throughout the Middle East," Boehner said Thursday.

Williams, pointing to the Democrats' letter, predicted that GOP leaders would deem it "pointless" to push hard to reject a finalized deal "given that the likely outcome would be a failed [veto] override vote."

"I've heard people say this could be the ObamaCare of foreign policy," he said, referring to the Republicans' repeated but unsuccessful efforts to repeal Obama's healthcare law. "But in terms of legislation that would achieve their goal of undermining the agreement, I think they're out of options."

This article was written by Mike Lillis for The Hill on May 9, 2015.