WASHINGTON — With Senate Democrats still struggling to line up support, the success or failure of President Obama’s four-month campaign to overhaul gun laws will most likely revolve around a single provision: a proposal to expand federal background checks for gun purchases.

Background checks, both advocates and independent researchers say, would have a bigger potential effect on gun violence than any other measure under consideration — including the much-discussed assault-weapons ban, which has little chance of passing in Congress. Proposed federal gun-trafficking laws and changes to mental health databases would have a marginal impact on gun violence, experts say.

But even though around 90 percent of those polled in public surveys support background checks, the fight for it and the rest of the first major piece of gun control legislation since 1993 faces a difficult test in the coming weeks. On Tuesday, Senate aides said that formal debate and substantive votes on the gun issues would probably slip to the week of April 15 — a setback considering that Senator Harry Reid of Nevada, the majority leader, had pledged that it would be the first issue to come up when Congress returns from spring recess next week.

Background checks are central to the delay. Efforts to reach a compromise between Senators Charles E. Schumer, Democrat of New York, and Tom Coburn, Republican of Oklahoma, have foundered. Separate talks between Senators Joe Manchin III, Democrat of West Virginia, andMark Steven Kirk, Republican of Illinois, have also yet to yield a breakthrough.

Among the most difficult questions are which gun-purchase records the government would track and which purchases would be subject to background checks.

“The background-check piece is the single most important thing we can do right now to make a difference,” said Pia Carusone, the executive director of Americans for Sensible Solutions, the advocacy group formed by Gabrielle Giffords, the former Arizona congresswoman, and her husband, Mark Kelly, after Ms. Giffords was shot in the head in Tucson in 2011.

“If the background check bill doesn’t pass Congress,” Ms. Carusone said, “we are not going to be happy.”

Mr. Obama will push for the legislation on Wednesday not far from the site of a movie theater massacre in Colorado last year. Next week, he will travel to Connecticut, where a gunman killed 20 children and 6 adults at a school in December, as part of a stepped-up public campaign to pressure lawmakers.

Lawmakers who support gun rights and officials with the National Rifle Associationderide background checks as ineffective and a threat to gun ownership. Most Republicans in the Senate oppose the measure, and several have said they will filibuster to prevent its passage. A handful of Democratic senators also have expressed doubts.

In the face of that opposition, approval of the legislation would represent a major victory for Mr. Obama and his allies, only months after gun-control legislation seemed a political impossibility. Advocates say that more stringent background checks would shut down large gaps in a system that allow felons, domestic abusers and the mentally ill to buy guns.

Since the existing background-check system began, in 1994, officials have screened more than 108 million people before they could buy a gun, according to the Bureau of Justice Statistics; the federal government has blocked 1.9 million attempted purchases because of felony convictions or other problems with the would-be buyers’ background.

But no background check is required for about 40 percent of gun purchases, including those made online or at gun shows, federal officials estimate. Requiring checks for those purchases would be the single most effective way to keep guns out of the hands of dangerous people, advocates say.

“The research is so strong that it would actually make a difference,” said Ellen Alberding, the president of the Joyce Foundation, which makes research grants to inform gun violence prevention efforts. “It has been our grantees’ top priority from Day 1 on this.”

Should it eventually pass the Senate, the background check legislation would then head to a House chamber firmly controlled by Republicans and advocates of gun rights. If it fails, gun-control supporters may have missed their best chance in a generation at significant new federal laws. For the president, it would be a reminder of the limits of his powers to persuade.

“We have always said that this would be hard,” said Jay Carney, the White House press secretary. “We remain engaged in conversations with the Senate and those senators who are interested in forging a bipartisan compromise on measures to reduce gun violence.”

A new assault weapons ban has always seemed the most visceral response to the massacre in Newtown, Conn. But with advocates of stricter gun laws turning their attention to background checks, opponents of new gun laws are doing so as well.

In the past, the N.R.A. had supported background checks. But in a February speech, Wayne LaPierre, the organization’s chief executive, lashed out at the tougher background checks being pushed by gun control advocates, which he called “the heart of their anti-gun agenda.” Mr. LaPierre said the legislation would create a national gun registry.

“When another tragic ‘opportunity’ presents itself, that registry will be used to confiscate your guns,” Mr. LaPierre warned.

Groups that oppose new gun laws are pushing hard to block the legislation. The Heritage Foundation’s lobbying group said Tuesday that it would consider votes on the gun proposals crucial to earning its support in the future. The group urged lawmakers to vote against what it called “feel good” legislation.

Gun control advocates are also stepping up their campaigns in support of an expanded background check system. The group headed by Ms. Giffords and Mr. Kelly will run online ads next week in four Western states — Arizona, Colorado, Montana and Nevada — urging people to call their members of Congress.

Separately, Mayors Against Illegal Guns, a group founded by Mayor Michael R. Bloomberg of New York, will continue to run television ads aimed at persuading senators to vote for the background check legislation.

John Feinblatt, a senior adviser to Mr. Bloomberg on guns, compared the existing background check system to an airport with two lines for screening passengers — one line with security and one where people “just waltz through” without being checked.

“If you were only going to do one thing, this is the single most important thing that Congress can do to prevent gun violence,” Mr. Feinblatt said. “That’s what is going to make a difference. That’s what’s going to keep people safe.”

Mr. Bloomberg has indicated he is willing to invest some of his personal fortune in defeating candidates who refuse to support gun control measures.

“If it does fail, then Congress is going to need to explain why they thwarted the will of the electorate,” Mr. Feinblatt said of the background check provision. “When the next tragedy occurs, members of Congress will have to explain what they did to prevent it.”


Jonathan Weisman contributed reporting.