WASHINGTON — As pressure mounts for congressional action on gun control, it’s unclear just how much can be done given the thick political fog that shrouds any major legislative effort on Capitol Hill — especially in an election year.

President Donald Trump tweeted Thursday that Congress “is in a mood to finally do something on this issue,” but lawmakers are at home on a weeklong recess and it’s too soon to tell if there’s enough appetite to tackle gun legislation when they return next week. Republican leadership in the House and Senate have been silent on the question.

A number of proposals to curb gun violence are floating around in Congress, some of which were started last year after a spate of other mass shootings.

But even with the new energy behind them, these proposals face an uphill climb in Congress. Here’s why:

1. Leadership in both chambers would have to support it
For any debate to seriously take place in Congress, it must first have the blessing of the top two Republican leaders: Senate Majority Leader Mitch McConnell of Kentucky and the House Speaker Paul Ryan of Wisconsin. They essentially control floor action and decide which bills get votes.

But so far, they have yet to signal how they’ll address this issue when they come back next week.

A day after the shooting last week that left 17 people dead at a Parkland, Florida high school, Ryan argued it was not the time to wage political battles as the country mourned.

“This is one of those moments where we just need to step back and count our blessings,” he told reporters Thursday at a news conference at the Capitol. “We need to think less about taking sides and fighting each other politically, and just pulling together.”

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Since then, however, students have taken the lead in protests and activism, marching to the White House, Congress, and the Florida state capitol, among other places across the country, to demand action. The images have kept the debate prominent in the media, indicating that Congress will return amid sustained pressure.

While Democratic leaders don’t have as much control over action on the House and Senate floor, they will also be important factors in just how unified their party will be on the issue. It’s true that the loudest pro-gun control voices are Democrats, but that doesn’t mean all members of the party in Congress want to address it. Some are from states or districts that Trump won easily and where gun control is not so popular.

Even when Democrats were in control of Congress and the White House in 2009 and 2010, they didn’t make gun control a priority. Sen. Bill Nelson, D-Florida, admitted Wednesday night in the CNN town hall that was a mistake: “Gun legislation under those circumstances should have been considered because there’d been a lot of massacres up to that point.”

2. The Senate requires 60 votes to pass most legislation
For almost all legislation to advance in the Senate, it must first acquire support from 60 senators on a key procedural vote before moving on to final passage, which only requires a majority. Such a rule is designed to make sure bills have at least some bipartisan support.

But the 60-vote threshold has thwarted major legislative efforts in the past, including when Congress made a big push on a gun control-related package in 2013, which followed the Sandy Hook Elementary School massacre. Despite some bipartisan support, a bill to expand background checks on gun purchases only managed to get 54 votes.

The current balance of power stands at 51 Republicans to 49 Democrats (or independents who caucus with Democrats). That means that even if all Democrats supported a gun control measure, they would need 11 Republicans to hit that magic number of 60.

There are already bills in the works. Republican Sen. Jeff Flake tweeted Wednesday that he’s working with Democratic Sen. Dianne Feinstein on a bipartisan bill to raise the age requirement to purchase a rifle to 21. Current federal law allows gun buyers to purchase a rifle or shotgun at the age or 18, while the age requirement for a handgun is 21.

Republican Sen. Marco Rubio of Florida said at a CNN town hall Wednesday that he also backs the idea of banning rifles and shotguns for 18-year-olds. “I will support a law that takes that right away,” he said. He added that he’s also reconsidering his support for large-capacity magazines. GOP Sen. Pat Roberts of Kansas reportedly made similar comments to the Kansas City Star.

On another front, Republican Sen. John Cornyn and Democratic Sen. Chris Murphy have a bill that would enforce better reporting of domestic abuse to the National Instant Criminal Background Check system. Trump spoke to Cornyn about the bill Friday, and since then, the President has twice tweeted about strengthening background checks.

3. The NRA remains a powerful lobby
The National Rifle Association has consistently objected to changes in gun laws in recent years, arguing that any further restrictions would amount to the breach of Second Amendment rights and lead to a slippery slope of more gun control.

While the NRA has been supportive of the Cornyn-Murphy bill that better enforces existing laws, it opposes raising the age requirement for rifles and shotguns. Instead, the NRA calls for a better mental health system and more security at places like schools to address mass shootings.

“I said five years ago, after that horrible tragedy in Newtown — and I wish, oh God I wish, more had heeded my words — so, lean in, listen to me now and never forget these words: To stop a bad guy with a gun, it takes a good guy with a gun,” said Wayne LaPierre, the executive vice president and CEO at the NRA, in a speech Thursday at CPAC.

While the NRA donates some money directly to candidates and campaign committees, it spends the vast majority of its money on lobbying and outside expenditures, like supporting or opposing candidates through powerful ads. For example, in the 2016 election cycle, it gave nearly $1.1 million directly to congressional candidates and party committees, but spent $54.4 million on outside spending.

It also wields influence with its report card system. The NRA grades candidates on a scale of “A” — “F”, with an “A” candidate being someone who’s made “a vigorous effort to promote and defend the Second Amendment,” while an “F” candidate is a “true enemy of gun owners’ rights.”

Depending on their politics, candidates often tout their rating from the NRA on the campaign trail. Some are happy to announce their endorsement from the NRA, while more liberal candidates freely tout their “F” rating.

Regardless, the NRA makes sure that its enormous membership knows where candidates stand on gun rights.

4. It’s an election year
Further complicating the first three obstacles is the fact that the entire House of Representatives and a third of the Senate is up for re-election this year, making any major legislative issue — let alone a divisive one like gun control — difficult.

Incumbents who face tough primary battles tend to retreat to their respective sides rather than engage in compromise. And those looking at a close race in the November general election will have a tough time casting risky votes.

Take, for instance, the immigration battle in the Senate that erupted earlier this month. Negotiators had been working for months on compromise legislation that would address the Deferred Action for Childhood Arrivals program, which technically expires in March. Despite two government shutdowns and fierce lobbying by immigration activists, the Senate couldn’t reach the 60 votes needed to pass a compromise bill. Nor could they get close on three other proposals.

Republicans, who control Congress, are eager to move on to safer bets like infrastructure so they can pass something to add to their records on the campaign trail.

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