New York City Mayor Michael Bloomberg delivers remarks at the Brooklyn Navy Yard on June 11. (Mario Tama/GETTY IMAGES)

When Congress in April defeated an effort to strengthen the national background-check system for gun sales, it was mostly on the strength of Republican opposition. Less than two months later, proponents of stricter gun laws have decided that a small number of Democrats will make more productive targets.

In the Senate, all but four Republicans opposed the background-check measure. They have emerged mostly unscathed by the various campaigns advocating for stricter gun laws in the wake of the December attack at Sandy Hook Elementary School in Newtown, Conn., that killed 26 schoolchildren and teachers.

With parents of the young Newtown victims providing emotional reminders of the shooting for lawmakers Wednesday, one of their most high-profile political allies, New York Mayor Michael R. Bloomberg (I), adopted sharper tactics, urging thousands of his city’s top political donors to withhold donations from four Senate Democrats who voted against the gun bill.

In a letter to more than 1,000 donors, Bloomberg called out the four Democrats — Sens. Max Baucus (Mont.), Mark Begich (Alaska), Heidi Heitkamp (N.D.) and Mark Pryor (Ark.). “Instead of rising above politics to pass a law that would save lives,” he wrote, the four senators “sided with a gun lobby increasingly out of touch with Americans’ priorities.”

“The next time these four Senators want you to support them with donations to their campaigns, tell them you cannot,” Bloomberg wrote.

By asking campaign donors to withhold funds, the deep-pocketed mayor went against the will of his congressional Democratic allies, who tried but failed to secure enough GOP support for the gun bill and have warned that public criticism of vulnerable Democrats who voted against the bill will result in Republican gains and less of a chance to enact new gun laws.

Since the bill’s defeat in April, Bloomberg’s anti-gun group, Mayors Against Illegal Guns, has spent hundreds of thousands of dollars targeting senators of both parties for voting against the measure. A group founded by former congresswoman Gabrielle Giffords (D-Ariz.) — who was severely wounded in a January 2011 shooting — also has raised millions of dollars as it airs radio and television ads.

Begich and Pryor face difficult races next year, while Heitkamp is a newly elected freshman and will not face reelection until 2018. Baucus has announced plans to retire. Spokesmen for Begich, Heitkamp and Pryor declined to comment, while a Baucus spokeswoman said that “campaign contributions have never and will never influence Senator Baucus’s decisions on any legislation.”

But Democratic control of the Senate is tenuous. Republicans are likely to need at least a five-seat gain to draw even or claim the majority, depending on how a pair of special elections this summer and fall play out. But the GOP has good chances of picking up open seats in South Dakota and West Virginia, and the outcomes of races in the targeted states of Alaska and Arkansas will be critical to control of the Senate in 2015.

Democrats stopped short Wednesday of directly criticizing Bloomberg. Sen. Michael F. Bennet (D-Colo.), who chairs the Democratic Senatorial Campaign Committee, said Bloomberg is targeting “very strong candidates.”

“Mayor Bloomberg is entitled to make choices he wants to make,” Bennet said at a breakfast hosted by the Christian Science Monitor. “He obviously feels very strongly about this issue and he has for a long time, and I’m not going to make a judgment about that. He’s entitled to do what he feels he needs to do.”

Sen. Joe Manchin III (D-W.Va.), who was lead sponsor of the Senate gun bill, said he would not “second-guess the mayor or any other group.” Manchin has received top ratings from the National Rifle Association and said he would rather see gun-control groups “trying to do something in more of an informational, educational and supportive role.”

Of the 35 Senate seats up for grabs in 2014, the 10 most contested battles come in mostly rural, conservative-leaning states such as Montana, West Virginia, Alaska and Arkansas, and nine of them are held by Democrats. Every Republican up for reelection in 2014, including the top GOP leaders, Sens. Mitch McConnell (Ky.) and John Cornyn (Tex.), have demonstrated that their biggest political fear is a primary challenge from the right, making them much more receptive to supporting the NRA’s position.

The long-term challenge for Bloomberg and other gun-control groups will be to keep the issue on the political front burner in the 2016 elections, when Republicans will face a much different Senate map and might be susceptible to arguments about gun safety.

In 2016, Republicans have to defend 24 of the 34 seats up for election, including a slew of seats in large states with huge urban and suburban populations where anti-gun forces have made big inroads: Arizona, Florida, Illinois, New Hampshire, Ohio and Pennsylvania.

But Republicans otherwise remain united by their position to filibuster legislation that would have expanded background checks on some non-retail sales of guns, and their House counterparts, firmly in charge of that chamber, have shown even less inclination to consider gun legislation than they were in the early spring.

This comes despite a vow from gun-control groups and President Obama — who warned that “this effort is not over” when the bill was defeated in April — that they would turn up the political pressure to make lawmakers change their positions and support the background-check proposal.

Gun-control groups united to rattle Sen. Kelly Ayotte (N.H.), one of the last Republicans to decide against supporting the measure, at town hall meetings in early May by placing victims’ advocates in the audiences to ask questions. Her statewide approval ratings took a modest hit after the vote.

They also targeted Sen. Jeff Flake (R-Ariz.), who said his approval ratings suffered because of his stance on guns. But he told reporters Wednesday that “I’m comfortable with the position I’ve taken” and has no plans to support a new gun bill.

Just ahead of Friday’s six-month anniversary of the Newtown shooting, some parents of the slain schoolchildren visited House Speaker John A. Boehner (R-Ohio) and other lawmakers Wednesday. Before the meeting, Boehner gave no indication he would bring gun legislation to the House floor, telling reporters only that “our hearts and souls go out to these families.”