This month, the Senate was expected to pass a bill to fight human trafficking and help victims of it. Most of the bill is overwhelmingly popular among politicians of both parties. And who would want to vote against a bill cracking down on sex traffickers?

Yet the effort now lies in shambles — because of controversy over one provision of the bill that blocks federal funding for abortions. Sen. Dick Durbin (D-IL) calls the provision "offensive language," and Sen. Dianne Feinstein (D-CA) has framed the fight over it as "a battle for our identity." Most of their Democratic colleagues have joined them, and have voted five times to filibuster the bill.

Republicans profess outrage. "Democrats actually filibustered a bill to help victims of modern slavery, apparently because left-wing lobbyists told them to," Senate Majority Leader Mitch McConnell has said. Now, McConnell's pledging that President Obama's nominee for attorney general, Loretta Lynch, won't get a confirmation vote until the trafficking bill is passed.

The particulars of the controversy are complex. But overall, the mess is yet another testament to the polarization of our politics in general, and the dysfunction of the Senate in particular. If Congress can't find a way to pass this slam-dunk bill, it's difficult to imagine them accomplishing much else of significance.

What does the anti-trafficking bill actually do?

The Justice for Victims of Human Trafficking Act is designed to improve US enforcement of anti-trafficking laws, more strongly penalize offenders, and help the victims of trafficking crimes. According to the Congressional Budget Office, the bill would:

Create a fine of $5,000 for people convicted of offenses related to trafficking or sexual abuse

Put those fines into a new Domestic Trafficking Victim's Fund, to be spent by DOJ on programs helping trafficking victims

Let lawful permanent resident victims of "severe forms of trafficking" be eligible for federal benefits (food stamps and Supplemental Security Income) more quickly

Require DOJ to better train enforcement officers and prosecutors working on trafficking

Create an annual DOJ report on how states are enforcing sex-trafficking laws

But there's one more thing: that new fund for trafficking victims? None of its money would be allowed to be spent on abortion or health coverage that includes abortion services, except in cases of rape, incest, or risk to the life of the mother. This is what's derailed the bill.

What's the controversy over this abortion section?

Republicans maintain that the provision is perfectly ordinary. They argue it simply reflects the existing Hyde Amendment, versions of which have regularly been attached to yearly appropriations bills for decades. The Hyde Amendment blocks federal funds from being used for abortion except in cases of rape, incest, or danger to the mother's life.

But most Democrats say they were unaware the provision was even in this year's version of the anti-trafficking bill. Democrats initially said the GOP "pulled a fast one" on them, but a staffer's mistake seems to have been at fault. A spokesperson for Sen. Amy Klobuchar (D-MN) eventually admitted that one of her aides was aware of the new provision but "did not inform the senator."

Whatever the reason, in February, the bill sailed through the Senate Judiciary Committee unanimously, and seemed headed for likely passage. But by mid-March, pro-choice groups finally noticed the provision, and began raising an outcry. Nearly all Democrats took up their banner, successfully voting five times since March 17 to filibuster the bill.

"Once advocates on both sides became actively engaged in this, it is difficult for either side to make a change"

Democrats and pro-choice groups argued that abortion access for trafficking victims was particularly important considering the sexual nature of many of these crimes. "The majority of human trafficking victims are women and girls, and they need access to the full range of reproductive health care services without barriers," Planned Parenthood president Cecile Richards said in a statement.

Democrats are also arguing that since the anti-trafficking bill isn't an appropriations bill, it would in fact be an expansion of how the Hyde Amendment has traditionally been adopted — it would be applied to a new pot of money (the fines on trafficking criminals) rather than traditional federal appropriations (which are funded mainly through taxpayer dollars). They also complain that the Hyde language would apply to this program for five years, rather than having to be renewed every year, as is currently the custom for appropriations bills.

A potential compromise could involve defining trafficking victims as rape victims, since abortions in cases of rape can be funded under the Hyde language. Already, the Daily Beast's Eleanor Clift wrote, "A source familiar with the language in the trafficking bill, and who supports it, concedes that the victims of trafficking likely could claim rape, and the language could be moot."

But for now, both sides are in a standoff over the provision. Senate Majority Whip John Cornyn (R-TX) says that removing the Hyde language "looks like we are not maintaining the status quo; it looks like it's an erosion." Democrats are similarly reluctant to cave. "Over the years, we have lost virtually every battle that has been on this floor, and we are tired of it. So we are taking a stand, and we are going to hold that stand," Sen. Feinstein said. "Once advocates on both sides became actively engaged in this, it is difficult for either side to make a change," Sen. Chris Coons (D-DE) told Susan Milligan of US News & World Report.

So the bill has been temporarily shelved, as the Senate is tackling a federal budget resolution this week.

What does this tell us about Congress?

Overall, despite the changeover to Republican leadership, it's clear that the upper chamber remains intensely polarized and dysfunctional, unable to get seemingly uncontroversial things done. The human trafficking debate is a case study in how the modern Senate — and our political system in general — works to block action:

First, there's the filibuster, which still trumps all. Fifty-eight senators want the human trafficking bill to advance — two short of the crucial 60-vote threshold. If not for the filibuster, this bill would be on its way to President Obama by now. Newly in the minority, the Democrats have quickly seized on the leverage the filibuster offers to try to extract concessions from the GOP.

Second, there's partisan polarization, both in general and on the abortion issue in particular. All 54 Republicans support advancing this bill, but just four Democrats do — Sen. Bob Casey (D-PA), Sen. Heidi Heitkamp (D-ND), Sen. Joe Manchin (D-WV), and Sen. Joe Donnelly (D-IN). The abortion issue is so polarized that it's turned a bill that got a unanimous committee vote into a partisan litmus test.

Third, and relatedly, moderates have declined in each party. In previous years, one could imagine a larger moderate faction of red-state Democrats concerned with burnishing their pro-life and moderate credentials. But there simply are fewer Senate Democrats from red states nowadays — after their 2014 losses, just five Democratic senators represent states Mitt Romney won in 2012.

Fourth, there are the outside groups. Both pro-choice and pro-life groups are watching this battle very closely and don't want their respective allies to cave on it. And this isn't unique to the Senate — in January, a pro-life provision derailed a bill in the House, when it was viewed as going too far by a group of GOP moderates.

All of these factors interact together. Polarization, the lack of moderates, and the vigilant outside groups make it very difficult for McConnell to pick up the six Democratic votes he needs to break a filibuster. And they make it clear that despite the Senate's new leadership, the chamber has a long way to go before it can be considered functional.