The effort by the gun reform movement to build a serious, long-term political infrastructure to challenge the NRA just got a boost this morning. The New York Times reports that Mayor Michael Bloomberg, who retains enormous clout in the financial community, will send a letter today to hundreds of top New York donors, urging them to stop giving any contributions to any Democrats who vote the wrong way on guns — angering Senate Dems who say it is a threat to their majority.

I’ve got a copy of the letter. It singles out four Senate Democrats who voted against the Manchin-Toomey compromise on background checks — Mark Pryor, Heidi Heitkamp, Mark Begich, and Max Baucus — and continues:

I am writing to ask you: the next time these four Senators want you to support them with donations to their campaigns, tell them you cannot. Until they show that they will stand up for the American people and not the gun lobby, tell them you cannot support their candidacy. These “no” votes were a slap in the face to Americans everywhere. Polls consistently show that 90% of Americans—including 82% of gun owners and 74% of NRA members—support requiring background checks for all gun sales. But instead of standing up for what’s right, and for a common-sense measure their constituents support, these Senators made a calculated vote designed to pander to the gun lobby in anticipation of their next election. [...]

Senators Baucus, Begich, Heitkamp, and Pryor sided with the minority, reminding us of why so many people are angry at Congress. Astonishingly, the four Senators did this even as they ask New Yorkers, who contributed a significant share of the funding for their last election bids, to donate to them once again. Their votes were an affront to the nation. Make no mistake: loopholes at the federal level undermine our efforts to keep our streets safe.

By voting against background checks, these Senators told us they would rather bow down to a special interest group than support a common-sense measure to help law enforcement fight gun crime. Now you can tell these Senators: by voting against background checks, they voted against your values and your family’s safety. And until they show they will stop bowing to pressure from the gun lobby, you should not support them.

The move, which was partly inspired by the recent decision by venture capitalist and Buzzfeed founder Kenneth Lerer and a few other top donors to withhold funds, is infuriating Senate Democratic leaders and operatives. According to the Times, Harry Reid privately suggested to Bloomberg that his efforts targeting Dems could result in making Mitch McConnell the Senate Majority Leader, which could set back the cause of gun reform even further.

But in the view of Bloomberg’s operation, Mayors Against Illegal Guns, and others who are trying to build a gun reform movement with some staying power, such considerations are beside the point. They view this as a battle that could take many years to win and will require a major shift in basic thinking about gun politics among many top Democrats. As top Bloomberg adviser Howard Wolfson told me the other day, gun control will not pass the Senate until most or all red state Democrats also support it, and that won’t happen until they decide that more of a price will be paid for voting against gun control than for it.

It is sometimes said that even if background checks are overwhelmingly popular in polls, that doesn’t really account for just how vulnerable red state Dems make themselves when they vote for something that can be caricatured as anti-Second Amendment. It’s also said that voting against gun control is an easy way for red state Dems to signal cultural affinity with their voters. But some red state Dems up for reelection in 2014 — such as Mary Landrieu and Kay Hagan — voted for Manchin-Toomey. If they don’t pay any price for that, will commentators take any message from it?

The gun reform forces increasingly believe that the above objections are founded on mistaken assumptions that need to be changed. They believe Democrats can explain a pro-background check vote to red state voters, even those inclined to be “pro gun” and “pro Second Amendment” — even if they are targeted by relentless NRA propaganda. They also believe these voters are less conservative on these issues than it’s often assumed. What’s really driving this effort is the idea that an understanding that this argument can be won – despite the NRA’s supposed omnipotence – has to take hold among more Dems for the gun reform movement to have long term success.

The counterargument, of course, is that the better route to success would be to do a better job targeting more vulnerable blue state Republicans to win them over, because the closer you get to 60, the harder it is for red state Dems to cast a decisive vote sinking gun reform. Increasing the Dem Senate majority, and perhaps reforming the filibuster, might also obviously be more effective means to the desired end. At any rate, Bloomberg’s move will likely force a debate about all of this.

 * SENATE DEMS DIVIDED ON IMMIGRATION: Politico reports that Senate Democratic leaders are divided over their immigration strategy, with some, such as Dick Durbin, questioning Chuck Schumer’s publicly stated goal of getting 70 votes in the Senate. As I reported here on Friday, Dems worry this could signal a willingness up front to give away too many concessions to the right in the quest for broad bipartisan support.

This whole thing could come down to whether John Boehner allows the final bill to pass with mostly Dem support. It’s unclear whether having broad bipartisan support in the Senate will make that more likely; the decision could be largely driven by the basic question of whether GOP leaders want reform to pass or not.

* GOP ESTABLISHMENT FIGHTS FOR IMMIGRATION REFORM: NBC News reports that a super PAC called Republicans for Immigration Reform, which is headed by several top GOPers, will enter the immigration fray. It will release new polling showing that even conservatives support reform, and more interestingly, it intends to run ads targeting House Republicans who might be hostile to citizenship. It’s another sign of just how much large swaths of the GOP establishment want reform to happen.

* PUBLIC CONFLICTED ON NSA SNOOPING: A new CBS poll finds that 58 percent of Americans disapprove of the federal government collecting the phone records of ordinary Americans, while only 38 percent approve. At the same time, however, 53 percent say the collection of such records is necessary to fighting terrorism, and only 36 percent say the government has gone too far in compromising privacy to fight terror, while a total of 59 percent say the balance is about right or hasn’t gone far enough.

In a sense this polling is useless, given how little the public actually knows about the scope, reach, and inner workings of these programs.

 * CLAPPER’S FALSEHOOD CONTINUES TO RESONATE: Post fact checker Glenn Kessler weighs in with his dismantling of Director of National Intelligence James Clapper’s suggestion that records are not gathered on millions of Americans. Again: If this program weren’t shrouded in such secrecy, this dishonesty wouldn’t have been required in the first place.

* WHY A BUDGET DEAL WITH REPUBLICANS IS IMPOSSIBLE: The Hill reports that top White House officials argued with Republican Senators at a closed door meeting over how to reach a deal on the budget. The key sticking point, tellingly, was over how serious the current deficit problem is.

Democrats are working off of a recent report from the nonpartisan Congressional Budget Office lowering its projection of the 2013 deficit to $642 billion. But as the Hill reports, Tea Party Senator Ron Johnson — who was key in the talks — brought his own numbers. One wonders what they showed and where they came from…

* ISSA IS FLAILING ON IRS INVESTIGATION: This is getting comical. When Sam Stein pressed Rep. Darrell Issa’s office to explain why it’s okay to release partial transcripts of testimony by committee witnesses in the IRS scandal, rather than full ones as Dems want, Issa’s office replied:

“Limited releases of testimony may also serve to empower other witnesses to become whistleblowers and serve to vindicate individuals who have been subjected to criticism or retaliation at the hands of their managers. On the other hand, if a full transcript were released, it would serve as a roadmap of the Committee’s investigation.”

Got that? Issa has already been pilloried by news orgs for falsely tying the IRS targeting directly to the White House, which tells you all you need to know about his true motives.

* LIBERALS PRESS DEMS ON CAMPAIGN FINANCE REFORM: The Progressive Change Campaign Committee, CREDO and other liberal groups are releasing a letter this morning to House Democratic leaders demanding that their Election Reform Task Force — which is currently deliberating several proposals to dilute the influence of big money on our politics — toughen up those ideas. The groups want it to focus on reform that would give public matching funds only to candidates who agree to only accept contributions from small donors. Read the letter here.

 * AND WHERE IS OUTSIDE MONEY IN MASSACHUSETTS SENATE RACE? Last night Dem Ed Markey and GOPer Gabriel Gomez had their second of three debates, with Gomez laboring to embrace Dem ideas as a way of shedding his ties to the national GOP on key issues. Here’s the thing to watch: In coming days, will the GOP-aligned super PAC start spending money on the race?

If so, that signals it has tightened. But they have yet to enter the contest, which suggests internal polling shows it may be less close than even public polls suggest.

What else?

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