The Gang of Eight's immigration bill has been introduced to the public, so it's time for Sen. Marco Rubio and company to roll up their sleeves and try to get it passed in Congress. The senators certainly sound confident: Chuck Schumer (D-N.Y.) and John McCain (R-Ariz.) have said that their goal is to get 70 or more votes in the Senate.

But is it really going to be that easy? This is, after all, one of the most ambitious pieces of immigration legislation in decades. (Get the details on the bill here.) This week, the bill faces its first hurdle when it goes to the Senate Judiciary Committee for a mark-up. A look at what else the bill is up against:

1. Jim DeMint and the Heritage Foundation
The former South Carolina senator is leading the charge against the immigration bill from his spot as president of the Heritage Foundation. On Sunday, he appeared on ABC's This Week to make his case:

The bill that's being presented is unfair to those who came here legally. It will cost Americans trillions of dollars... Heritage is the only organization that has done an analysis of the cost. Unlawful immigrants make up about 2 percent of our GDP, and they consume most of that. If you consider all the factors of amnesty and unlawful immigration, the cost will be in the trillions of dollars over the lifetime of these unlawful immigrants. [This Week, via Heritage Foundation]

The foundation's opposition could put a damper on the bipartisan glow this bill has enjoyed since the beginning by giving its opponents some common talking points to take to the media. (Update: Many Republicans are pushing back on the Heritage report, dinging it as factually inaccurate.)

2. Tea Party-backed senators
The Republicans in the Gang of Eight — Rubio, McCain, Jeff Flake (R-Ariz.), and Lindsey Graham (R-S.C.) — seem convinced, at least in public, that they can get a majority of their party in the Senate to play ball. That doesn't mean the bill doesn't have its detractors.

Sens. Rand Paul (R-Ky.), Ted Cruz (R-Texas), and Mike Lee (R-Utah) — Tea Party favorites — were the ones threatening to filibuster during the gun-control debate. Now, according to Breitbart, they are the most prominent senators joining Jeff Sessions in his battle against the Gang of Eight's bill.

"There are so many problems with this bill, it is hard to know where to start," said Sessions, according to The New York Times. He claims that the legislation would drastically increase immigration to the United States and hurt the U.S. job market, especially for low-skill workers.

Sessions and his colleagues will have plenty of opportunity to sway public opinion, too. According to the Pew Research Center, nearly four in 10 Americans don't know enough about the immigration bill to say if they support it or not. 

3. House Republicans 
Even if the bill waltzes through the Senate, it still has to pass the House — something that Rubio told Sean Hannity last week probably couldn't happen with the bill in its current form.

And some House Republicans in Congress aren't even waiting for the bill to make its way through the Senate or for a similar version to be introduced in the House. Rep. Bob Goodlatte (R-Va.), chairman of the House Judiciary Committee, wants to introduce immigration legislation piecemeal in what The Washington Post's editorial board is calling a delay-and-dismemberment plan:

That strategy gives conservatives a chance to say they were for immigration reform before they were against it. They may vote for bills that would tighten border security, provide a steady source of migrant farm workers and expand a program that companies may use to verify the immigration status of employees. Then, decrying 'amnesty,' they can shoot down measures that would extend legal status and eventual citizenship to most of the undocumented. [Washington Post]

Striking down an eventual path to citizenship would be a no-go for Democrats, which could create enough friction to derail immigration reform all together.

4. The split over green cards for same-sex couples, and other amendments
As I wrote last week, an amendment to the bill allowing for American citizens to sponsor their foreign-born same-sex partners for green cards has caused a rift in the Gang of Eight. Rubio is worried that all of the bipartisan goodwill in the Senate could disappear over the divisive cultural issue. President Obama has said that while he supports the amendment, he would sign a bill that didn't include it, saying, "I'm not going to get everything I want in this bill, and Republicans are not going to get everything they want."

That amendment is just one of many to be debated by Republicans and Democrats in the Senate. Others, according to Politico, include tighter border security standards, quicker and longer pathways to citizenship, an increase in family-based visas, and much, much more. Any one of them has the potential to fracture the tenuous alliance built by the Gang of Eight, sending the whole thing into partisan chaos.