WASHINGTON - Calling for "a national movement" on gun laws, Senate Democrats on Thursday urged the public to build pressure on Congress to tighten background checks and fight the illegal gun trade.

"We are now asking - as the president said - for the people to make their voices heard," said Sen. Charles Schumer (D., N.Y.). "We expect there will be a groundswell."

Framing their effort as a response to last week's shootings at an Oregon community college and other massacres, more than two dozen Democrats stood on the Capitol steps pleading for a popular movement to reignite an issue that, despite more killings, has grown stagnant.

But with the Republicans who control Congress almost unanimously opposed to new gun-control laws, the effort's immediate impact is more likely to be political: drawing a contrast between the parties and ramping up pressure on GOP lawmakers facing tough reelection battles - a list perhaps topped by Pennsylvania Sen. Pat Toomey.

In 2013, Toomey won praise as a rare Republican who crossed party lines on guns when he cosponsored a bill to expand background checks, written after 26 children and teachers were shot to death at the Sandy Hook School in Connecticut.

He has trumpeted the effort as an example of his willingness to compromise in a dysfunctional Congress, and was honored this summer by a group of Sandy Hook families.

In the months since, Democrats who hope to challenge him next year have tried to chip away at his credentials, questioning his commitment to the issue because he has not reintroduced the background-check bill.

"If you are truly committed to doing the right thing - and you truly believe in your efforts - you do not let failure deter you," Katie McGinty, one of three Democrats in the race, wrote to Toomey on Monday in the latest salvo.

Another potential Democratic rival, Joe Sestak, has made similar comments, and the third candidate, Braddock Mayor John Fetterman, also supports tighter gun laws.

At his own event Thursday - introducing a bill to increase the penalties for criminals who attack police - Toomey said he and other Republicans had not been not consulted or invited to the Democratic gun-control rally.

"In order for this to be successful, and that's my goal, to actually have some success, it's going to have to be balanced, it's going to have to be bipartisan," Toomey said, noting that in 2013 his background check plan was introduced with Sen. Joe Manchin (D., W.Va.) and included new protections for lawful gun owners. "I continue to fully support that approach."

Their bill was ultimately blocked by Republicans and a handful of Democrats from conservative states. The issue has since lain dormant in Congress.

The shooting deaths of nine people in Roseburg, Ore., brought it racing back to the forefront.

A few minutes after Toomey spoke and a short walk away, Manchin joined fellow Democrats pleading for change. Senators from Connecticut and Virginia described mass shootings in their home states as the worst days of their lives.

"Today, we call on Americans from Roseburg to Blacksburg to speak with one voice - end the complacency," said Sen. Ron Wyden (D., Ore.), referring to the student killings in his state and in 2007 at Virginia Tech.

They did not offer specific legislation, but said they would write bills once public pressure mounts. Their principles, Schumer said, include closing loopholes in background checks, expanding gun bans on domestic abusers, and strengthening penalties against gun trafficking.

GOP leaders dismissed the Democrats' call.

"The president told them to politicize the issue, and apparently they're taking him up on it," said a spokesman for Senate Majority Leader Mitch McConnell (R., Ky.).

Most Republicans argue that expanding gun laws would infringe on Second Amendment rights and fail to prevent gun violence. They say the focus should be on mental health.

Polls show strong support for tougher background checks. But experts on the politics of gun laws doubted this push would lead to new laws when even the Columbine and Sandy Hook shootings failed to yield results.

"It's hard to see how the calculus has really changed," said Donald P. Haider-Markel, a University of Kansas professor who has studied public opinion on gun laws.

"I would assume that their chief goal is try to focus attention on the issue to indicate that Democrats in the Senate have a desire to try and move ahead," said Robert Spitzer, a State University of New York-Cortland professor who has written books about the politics of gun control. "They must feel that there's some electoral benefit to be had."

Toomey said Thursday he would "take a look" at Democrats' proposals. He has repeatedly said he still supports his background check measure, but that he doesn't see a way to advance it in a Senate that has grown more conservative since 2013.

Democratic caucus spokesman Matt House said if Toomey "supports our package - and would bring with him the many Republican votes we'll need to pass it - we would welcome his support."

But in a sign of how delicate the politics are, Manchin, Toomey's cosponsor in 2013, said he didn't know if he would support anything beyond the plan the two already had proposed.

"It's as relevant today, if not more so, as it was at that time," he said.

Manchin said that he continues to talk to Toomey about the bill, and that they would like to reintroduce it, but he could not say when that might happen.

As he prepares for his first Senate reelection, Toomey faces a difficult balancing act. He is running in a swing state that leans left in presidential years. But he also has to worry about conservatives who make up the Republican base and fiercely oppose gun control.

One Pennsylvania group that backs gun owners' rights has questioned Toomey's stand and threatened protests if he brings back the background-check bill.

"I frankly don't know of any Second Amendment organization that would be able to support him," said Tom Campione, legislative director of Pennsylvanians for Self Protection.

@JonathanTamari