Sen. Dianne Feinstein (D-Calif.) introduced a new version of the assault weapons ban on Thursday.

And it's already looking like a lost cause.

Most everyone agrees that the ban is the most ambitious and politically difficult item on President Obama's and Vice President Biden's gun control agenda. And there is increasing evidence that it will be cast aside in favor or more doable proposals. Below, we look at four reasons why.

1. Joe Biden is downplaying it

The same day that Feinstein introduced the bill, Biden suggested that magazine sizes were the most important part of a gun control package. "I'm much less concerned, quite frankly, about what you call an assault weapon than I am about magazines and the number of rounds that can be held in a magazine," Biden said in a Google Hangout. He added that "more people out there get shot with a Glock that has cartridges in a (high-capacity magazine)" and also suggested that shotguns are more deadly than so-called assault weapons. Biden then again downplayed the ban again during a two-hour roundtable discussion on Friday.

This, we remind you, is all within 24 hours of the assault weapons ban being introduced by a Democratic senator. And Biden is already giving us reasons why it's not that big a deal.

Biden is the point man on all of this. His words matter, and he's quite aware of the current politics of his issue. The fact that he's downplaying the assault weapons ban suggests that it's not likely to happen and he doesn't want the whole thing to be viewed as a failure if the ban isn't passed.

2. The votes aren't there

As Bloomberg reports, red-state Democratic Sens. Max Baucus (Mont.), Jon Tester (Mont.), Mark Begich (Alaska), Heidi Heitkamp (N.D.) and Joe Manchin (W.Va.) have all suggested they won't support an assault weapons ban, as have independent Sen. Angus King (Maine) and moderate Republican Sen. Susan Collins (Maine).

And that list doesn't include some other red-state Democrats who are up for reelection in 2014, including Sens. Mary Landrieu (La.), Mark Pryor (Ark.) and Tim Johnson (S.D.).

Quite simply: passing an assault weapons ban in the House is difficult enough. If even a few Senate Democrats bolt, Republicans will suddenly have no incentive to cast risky votes, and there is no hope for the bill. And it's very hard to believe that all of these red-state Democrats are suddenly going to come together and vote for an assault weapons ban.

3. Time is the enemy

Every day that the country gets further and further away from the tragedy in Newtown, Conn., the Obama Administration's quest for new gun measures becomes more difficult.

While there was arguably more impetus for gun control after Newtown than there has been in years, it's clear that the momentum has waned to some extent and people have, as the always do, refocused their attention on other political things, like illegal immigration, the budget and Obama's Cabinet nominations.

Obama knew this was a possibility, stressing in the days after Newtown that the Administration wouldn't simply ignore the recommendations of Biden's new gun control panel, as has happened with other special commissions. Obama made good on his promise, but in the intervening weeks, the immediacy of the issue was lost and senators, predictably, became much less willing to go out on a limb politically.

4. Obama and Biden don't need the ban

Even if the final result includes universal background checks and some of the other items, with no assault weapons ban, Obama and Biden can still lay claim to the most significant gun legislation in years/decades. They -- and the red-state Democrats mentioned above -- can still say they did something to avert future massacres and feel good about their efforts.

Whether it will be seen as a victory or not will depend a lot on how they prepare the American public for the outcome. After all, it's far better to under-promise and over-deliver than vice versa.

When it comes to the assault weapons ban, Biden and Democrats like Feinstein are setting expectations somewhere between "not going to happen" and "unlikely." And that's probably about where they should be.