President Obama is set to unveil a sweeping set of gun control proposals Wednesday, including a ban on assault weapons.

The problem for gun control advocates who’ve long called for such a measure is that there are fresh signs that it could be stonewalled on Capitol Hill.

For starters, two high-ranking congressional Democrats have expressed some degree of pessimism in recent days about the prospects of passing a ban in Congress.

Senate Majority Leader Harry Reid (D-Nev.), a gun rights advocate, said he was uncertain whether a ban could pass the Senate, and even less optimistic that it could clear the Republican-controlled House.

"Is it something that can pass the Senate? Maybe. It is something that can pass the House? I doubt it,” Reid said Friday on “Nevada Week in Review.” Reid has received a “B” rating from the National Rifle Association.

Senate Majority leader Harry Reid (D-Nev.) has expressed doubts that an assault weapons ban could pass Congress. (Susan Walsh/AP)

On Tuesday, House Minority Whip Steny Hoyer (D-Md.) did little to refute Reid’s suggestion that an assault weapons ban would run into trouble in the House, telling reporters: “I don’t have an assessment right now, I think that’s been the case based on past history.”

While the outlook in Congress is murky, it’s clear that most Americans favor such a ban. A Washington Post-ABC News poll released on Monday showed 58 percent of adults and 59 percent of registered voters support a nationwide ban on the sale of assault weapons. A Pew poll released the same day showed 55 percent of Americans support a ban.

That may sound like good news for gun control activists. And in some ways, it is. But a closer look at the numbers should trouble advocates of an assault weapons ban.

There are notable gaps between Democrats and Republicans on the issue, the polls show. In both surveys, clear majorities of Democrats favor a ban, while narrow majorities of Republicans do not.

In the Washington Post-ABC News poll, 76 percent of Democrats said they support a ban, while just 45 percent of Republicans said they do. In the Pew poll, 69 percent of Democrats said they support a ban on assault style weapons, while just 44 percent of Republicans said the same thing.

The divide matters, especially in the House, where many Republicans hail from safe GOP districts in which the threat of a primary from a more conservative candidate looms larger than the possibility of losing in the general election. Plus, the House GOP Conference has already demonstrated its stubbornness when it comes to issues demanding compromise from both parties.

There is much broader consensus for requiring background checks for sales of guns at gun shows and in other private transactions, to end the so-called “gun show loophole.” Nearly nine in ten Americans in both polls support such background checks.

Why does this matter when it comes to an assault weapons ban? Those opposing it might nonetheless favor new background checks, especially since the public is overwhelmingly supportive of the latter. And if they come to the table with something, they could evade attacks from opponents that they are doing nothing to impose new restrictions.

To be clear, it’s not a certainty that an assault weapons ban will fail in Congress. Sen. Dianne Feinstein (D-Calif.) has said she will soon introduce such bill in the Senate, and gun control advocates have been ramping up an effort to get the public on their side. Continued vocal support from Obama would further bolster their cause.

Obama will propose specific details via executive order Wednesday, and what specifics he will call for legislatively are not yet clear. What is clear is that an assault weapons ban has become a focal point in the debate over guns, and at this point at least, it has a ways to go before it could win passage from Congress.