Prime Minister Theresa May might sneak a very tight victory on Tuesday, but an array of parliamentary blocs opposed to her deal mean the odds are against her.

That's my basic assessment of what will happen at 3 p.m. ET when members of parliament in Britain's House of Commons vote on May's Brexit deal. That deal would govern a transition period following Britain's planned March 29 departure from the European Union. But, while I think May has the best deal she could get from the EU, many in parliament disagree.

First off, there are the "hard-Brexit" fundamentalists of May's own Conservative Party. Unified under the European Research Group parliamentary caucus, these MPs believe May's deal offers insufficient break with the EU. They say that May's deal would keep Britain overly tied to various EU regulations, especially in the area of trade and customs. The ERG have shown they are willing to act where it matters: They were instrumental in defeating May on this same deal back in January. While the prime minister's team had hoped that some ERG would shift their voting intention in light of new concessions granted by the EU late on Monday, they have since ruled out doing so. That said, May will have her fingers crossed that some of these MPs will abstain rather than vote against her.

The next bloc against May are the so-called "remainer" MPs who oppose Brexit. Wishing to overturn the referendum that precipitated Brexit, these MPs believe that a new referendum should be called. They believe that the public will then vote to remain inside the EU.

Next up, there are the erstwhile allies of May's government from the Northern Irish DUP and other Conservative MPs outside of the ERG. While these MPs support May on other policy issues, they have rejected May's Brexit agreement in the belief that it offers too few protections against a breach in British sovereignty. Their particular concern fixes on the so-called "backstop" arrangement that would govern the Northern Irish border with the Republic of Ireland in the event no final status deal was reached with the EU.

Finally, there's the opposition Labour Party. Led by Jeremy Corbyn, Labour wants May to call a new election that might allow them to enter power and then strike their own deal with the EU. But Labour has also suggested that it might call a second referendum if May's plan is defeated on Tuesday. It's worth noting here that Labour is nearly as divided on Brexit as are the Conservatives.

Regardless, the simple conclusion of these various interest groups is that May's plan looks likely to be defeated. If that happens, Brexit itself may fall by the wayside. It's a big day.