As support grew yesterday for Rep. John D. Dingell's efforts to weaken a Senate plan for background checks at gun shows, House Democrats confronted a sobering reality: One of their own may have just robbed them of a potent issue in next year's elections.

The National Rifle Association said it prefers Dingell's plan for 24-hour background checks at gun shows, and lawmakers in both parties predicted the proposal will likely be adopted when the House takes up gun legislation Thursday.

That prospect infuriated House gun control advocates, who see the proposal as considerably weaker than a Senate plan giving dealers 72 hours to conduct background checks on purchasers at gun shows. And Democrats fumed that the most senior Democrat in the House had effectively killed off the chances of enacting broader gun control measures, an issue President Clinton and other party leaders hoped would be a centerpiece of congressional campaigns next year.

Minority Leader Richard A. Gephardt (D-Mo.) privately urged Dingell not to offer his alternative, and Rep. James P. Moran (D-Va.) described Dingell as "doing the NRA's bidding."

"As the senior Democrat, he provides cover for a lot of members whose constituents are torn on guns," Moran said. "The Democrats are going to wind up getting credit for weakening gun control and the Republicans are going to get credit for strengthening gun control."

But Dingell, who was perhaps the most widely feared lawmaker in Washington when he served at the helm of the Energy and Commerce Committee between 1980 and 1994, appeared impervious to such criticism as he met with reporters yesterday. Dingell recounted how he had undergone a background check in Maryland this month when purchasing "a wonderful little shotgun" and compared gun shows to "what the pilgrims did when they had fairs in the early days of the United States."

"I have heard nothing from any Democrat indicating they disapprove of what I'm doing," said Dingell, a lifelong hunter and gun proponent who served on the NRA board until resigning in 1994. "This is an amendment which closes loopholes, which gives us a good, workable piece of legislation and does not create a situation where most sportsmen will feel they are being improperly inconvenienced by the bill."

Dingell's maneuvering -- which included meetings with House Majority Whip Tom DeLay (R-Tex.) -- clearly pleased Republicans, who have bridled at Democratic efforts to portray them as obstructionist on gun control. "It shows that people on both sides of the aisle think the problem is much deeper than some superficial treatment of guns as the solution," said Chief Deputy Whip Roy Blunt (R-Mo.). It also exposed the Democrats' divisions on gun control; lawmakers said as many as 60 Democrats could defect with Dingell, further undermining party leaders' hopes of drawing sharp distinctions with the Republicans.

Only yesterday, Clinton renewed his call for Congress to pass a tougher gun measure than proposed by Dingell. "The heart and soul of America is on the line," Clinton said.

Prospects remained murky yesterday not only for assorted gun measures, but also for the anti-violence measures that will be considered today as part of a juvenile justice bill that gained momentum after the Littleton, Colo., high school shootings.

The Rules Committee announced yesterday that the House will consider measures to crack down on the entertainment industry and juvenile crime today, to be followed by the gun proposals on Thursday.

Late last night, the committee approved floor debate on 55 amendments to the bills. "Virtually every proposal which has been put before us will be fully debated," said House Rules Committee Chairman David Dreier (R-Calif.).

But Democrats protested the voting procedure established by the committee, complaining that the rule prohibited an up-or-down vote on a Democratic substitute that, among other things, would have implemented all three of the Senate-passed gun control measures, one of several approaches for keeping guns away from juveniles that has support in the House.

The Senate bill, which many House Democrats have embraced, calls for 72 hours to conduct background checks at gun shows, mandatory child safety locks on guns and a ban on the importation of large-capacity ammunition clips.

House Judiciary Committee Chairman Henry J. Hyde (R-Ill.), by contrast, has drafted a plan that also would impose 72-hour background checks but narrow the range of gun shows where such checks must be conducted.

Dingell's plan would limit the scope of the background checks in other important ways. For instance, it would only require dealers to take 24 hours for a background check at gun shows, a proposal the NRA said would make it more convenient for lawful purchasers to buy weapons. Otherwise, a lobbyist said, at least a third of people buying weapons at gun shows could be denied access to weapons because of having to wait three days for clearance.

Such provisions have won the Dingell plan the nod of the powerful rifle association. "We simply made suggestions to Mr. Dingell's staff as to problems with the underlying McCollum-Hyde bill," said NRA chief lobbyist James J. Baker, adding that the final product addresses all of his group's concerns and is "much more reasonable" than the Hyde version.

Many Democrats questioned why Dingell, a veteran liberal and union loyalist from the Rust Belt who stands to regain his chairmanship if his party wins the majority next year, would forge a legislative alliance with DeLay, whom one Democrat privately described as "our archenemy." But they also noted he seemed haunted by the devastating loss then-Judiciary Committee Chairman Jack Brooks (D-Tex.) faced after backing gun control in 1994.

Spokesman Dennis Fitzgibbons said Dingell is "very concerned with expanding the universe of Democrats in the House."

CAPTION: The NRA likes Rep. John D. Dingell's background check amendment.

CAPTION: Withhold amendment, Rep. Richard A. Gephardt (D-Mo.) urged Dingell.