Sen. Charles Schumer (D-N.Y) choked up during a press conference on immigration reform Thursday, as he talked about the late Sen. Edward Kennedy’s (D-Mass.) influence on the issue. (The Washington Post)

In the strange political calculus that is Washington, the death of gun-control legislation in the Senate this week could boost the prospects of the next big contentious issue on the docket: immigration reform.

“Both of them provoke strong, emotional responses, and so I guess to the extent that one of them is moved to the sidelines, that provides some modest relief,” Sen. John Cornyn (Tex.), the No. 2 GOP leader, said Thursday.

But senators and senior advisers in both parties said the gun legislation’s fate was always closely tied to an immigration reform bill — and even to same-sex marriage, another high-profile issue senators confronted when they returned from their spring recess two weeks ago.

Some senators, faced with these difficult issues, feel the need to ration their political capital and courage and limit the amount of voter outrage they are willing to incur.

Many senators could afford to support one, maybe two, but not all three, said Sen. Joe Manchin III (D-W.Va.), the lead negotiator of a failed bipartisan plan to expand background checks on gun purchases.

“It takes a lot of work and effort,” he said Thursday, referring to the pileup of contentious issues on the Senate calendar, “and probably someone is saying to them, ‘Do you want to take on two or three?’ ”

Some senators from conservative states, particularly Republicans, decided that it would be too politically risky to support both gun control and immigration reform. They worried, aides said, that doing so would invite conservatives to challenge them ahead of the 2014 midterm elections.

Although just four Republicans supported the key provisions of the gun bill, far more GOP lawmakers are expected to back immigration legislation. After Republican presidential candidate Mitt Romney’s anemic showing among Hispanic voters in November — 27 percent — many in the party feel a more urgent need to reach out to voters in that demographic, and are using the immigration issue to do so.

Despite the optimism, proponents of immigration reform face a difficult political fight against determined and well organized opponents.

The gun legislation that was defeated on Wednesday was a 49-page bill that mostly sought to tweak procedures for buying firearms, and it was written by two pro-gun senators with deep ties to the National Rifle Association. Still, it was swallowed up in a senatorial quagmire.

The immigration bill, by contrast, is an 844-page document that would revise the nation’s immigration rules and regulations.

“It’s an indication that a lot of favorable publicity on a subject doesn’t always result in passage of a bill on the subject,” said Sen. Jeff Sessions (R-Ala.), an opponent of the gun-control and immigration proposals. “Immigration may not have as much support as it appears today. That’s sort of similar to the gun bill.”

The quandary facing the guns-and-immigration debate played out over the past several days, as Manchin and Sen. Patrick J. Toomey (R-Pa.) tried to find support for their compromise amendment for expanded background checks. Senate leaders agreed that all amendments receiving 60 votes would pass, and the Manchin-Toomey measure stalled at 55 votes with a handful of undecided senators left.

Among the Republicans still deciding were Sens. Jeff Flake (Ariz.) and Dean Heller (Nev.), both from states with booming Latino populations. Flake was a member of the Gang of Eight that crafted the bipartisan immigration proposal, despite a vocal anti-immigration bloc in Arizona. Heller, who was not a member of the group, said he was “optimistic we can find solutions and address these issues once and for all” after the group released its immigration proposal.

Both opposed the Manchin-Toomey amendment. Flake voiced his opposition a day before his group announced its immigration bill, and Heller just a few hours beforehand.

Sen. Heidi Heitkamp (N.D.), one of the final Democrats to decide, won election in November in a socially conservative state that favored Romney by more than 20 percentage points over President Obama. Less than two weeks before the votes on the gun legislation, Heitkamp joined the growing number of Democrats endorsing same-sex marriage, just days after her state approved the nation’s strictest law curbing abortion rights.

A couple of hours before Wednesday’s gun votes, Heitkamp announced her opposition in siding with her state’s pro-gun culture.

Each of the senators cited technicalities in the bill that, they said, would infringe on gun owners’ rights.

A handful of Democrats facing difficult 2014 elections also held their powder until the final days or hours before the gun vote. Three voted no. Of the five, Sen. Kay Hagan (D-N.C.) is the only supporter of the Manchin-Toomey proposal and same-sex marriage legislation.

Some senators saw a combustible mix of too many social issues colliding.

“There is a lot happening right now that, obviously, many people are very sensitive to,” said Sen. Bob Corker (R-Tenn.).

Corker sided with those who believe that immigration will find more support because its substance is far reaching. The first of what is expected to be a month of hearings in the Judiciary Committee will begin Friday, with full Senate consideration in June.

“The scope of immigration is much bigger, the debate will be much more real, because the policies are far more substantial,” he predicted.

Initial estimates from GOP advisers suggest that more than half of the Senate’s 45 Republicans favor the immigration plan, which would seem to give it a much clearer route to passage.

Yet immigration opponents will have many opportunities to pick away at such a large legislative target. “For those of us that were here in 2007, this is a lot more complicated than it appears at first blush,” Cornyn warned.

He was referring to the last immigration overhaul effort, when he and about a dozen fellow Republicans worked on a compromise plan to blend tougher border security with a pathway to citizenship for some of the illegal immigrants who have been in the United States for years.

It fell apart amid a sea of politically charged amendments that opponents offered, designed to win approval but also sink the overall legislation. There is already talk of amendments that would grant a visa to the same-sex partner of someone on the path toward citizenship, an idea Democrats champion but one that might torpedo any Republican support for the overall measure.

“Things like that would take the bill in the wrong direction. Immigration’s hard enough,” Sen. Lindsey O. Graham (R-S.C.), one of the immigration negotiators, said recently. “Let’s [not] go down to redefining marriage, providing more abortions. This is hard enough by itself.”

Manchin said that on gun legislation or immigration or other social issues, more senators are going to have to give ground.

“I think there’s not a whole lot of people really getting out of their comfort zone,” he said.

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