As I’ve been reporting, there’s a rising sense that if House Republicans don’t act by summer on immigration reform, the window for action could close for good. If nothing happens by August recess, the pressure on Obama to act unilaterally could become overwhelming, and any executive action will likely make legislative reform even less likely, perhaps postponing it until at least 2017.

Now even Republican-aligned constituencies who want reform are concluding the same thing. They are growing increasingly alarmed that they are at risk of getting cut out of the process — and their interests badly damaged — as the best chance to reform the immigration system in years is now in serious danger of slipping away for the foreseeable future.

Craig Regelbrugge, who co-chairs the Agriculture Coalition for Immigration Reform, says a large majority of his group’s members — which include large and small farming enterprises and growers all around the country — are Republican, and many give to the GOP. But he’s increasingly hearing from members who are so frustrated by the Congressional GOP’s failure to act on reform — which is central to maintaining a workforce in the industry — that they are considering withholding campaign donations.

“I hear from growers frequently who basically say, `I used to be a loyal check writer when the Republican Party called, but at this point, the checkbook is closed,’” Regelbrugge tells me. “I’m hearing from growers who are no longer writing checks supporting the party.”

Mike Gempler heads the Washington Growers League, which represents growers ranging from mom-and-pop outfits to enterprises spanning 10,000 acres, and he says that “well over 90 percent” of his members vote Republican, and many write checks. Some of them sit in the district of GOP Rep. Cathy McMorris Rodgers of Washington State, a member of the GOP leadership.

But, he says, they are increasingly convinced the GOP is no longer representing their interests in the immigration debate, if the failure to move on legislation is any indication, and are concluding that Republicans are very close to squandering a rare opportunity to achieve reform.

“We’re seeing a lack of response to our needs and concerns from significant parts of the Republican caucus in the House,” Gempler tells me. “They either have ideological issues or they are catering to a more reactionary crowd.”

“We want to see the leadership, including Cathy, move on this,” Gempler continues. “The chances for getting immigration reform are lessening quickly. If we don’t get this done by August recess, we’re going to be in trouble as an industry.”

And Tom Nassif, the president of the Western Growers Association, which represents hundreds of farmers in California and Arizona, told the New York Times that the Republican Party was at risk of losing the agriculture industry:

After the 2012 presidential election, as Republicans spoke enthusiastically about the need to court Latinos, Mr. Nassif was optimistic that immigration would become a top priority. But exasperation has replaced his confidence in recent months, and he said his group could withhold hundreds of thousands of dollars in congressional races in which it has usually supported Republicans.

“I can tell you if the Republicans don’t put something forward on immigration, there is going to be a very loud hue and cry from us in agriculture,” Mr. Nassif said. “We are a tremendously important part of the party, and they should not want to lose us.”

The Times piece, by the way, does a nice job of explaining why this is such a critical issue for this GOP-aligned constituency.

All this gets to a point about the immigration debate that keeps getting lost: Major Republican-aligned groups want reform — from growers out west to the business community to to evangelicals — and when Republicans refuse to act because they fear blowback from anti-reform conservatives, they are prioritizing them over other core constituencies. Now the growers are increasingly convinced the chance for reform is slipping away and they are getting cut out as a result.

GOP-aligned groups were, for a time, optimistic that reform was a real possibility. The GOP consultant class was taking the demographic threat seriously. A coalition of center-right groups seemed in sync and engaged. A compromise Senate bill passed with broad bipartisan support. John Boehner and Paul Ryan genuinely seemed ready. Boehner’s dust-up with conservatives after the government shutdown suggested he wouldn’t let hard-liners set the immigration agenda. GOP reform principles showed a real break with hostility to any form of legalization.

But then came the blowback from the right, and Boehner cowered. Now GOP-aligned interests are watching as GOP leaders seem unwilling to engage with the reality that Obama may have no choice but act alone if they don’t — and that this could scuttle reform’s chances for good.

“We are deeply concerned that Obama acting unilaterally does poison the well,” Regelbrugge of the Agriculture Coalition for Immigration Reform tells me. “On the other hand, the best prevention strategy to the well getting poisoned is for Republicans to lead the way and tackle reform. Hope is waning.”

Greg Sargent writes The Plum Line blog, a reported opinion blog with a liberal slant -- what you might call “opinionated reporting” from the left.