House Republicans' decision to double down on their demand that any measure to keep the government open must be linked with the defunding of President Obama's health care law makes a shutdown at the end of this month increasingly likely.

The Capitol dome is seen on Capitol Hill in Washington, Wednesday morning. April 6, 2011(AP Photo/J. Scott Applewhite)

Here's why -- in 5 steps.

1. Speaker John Boehner had a choice going into today's meeting: Go forward with a plan similar to the one that House Majority Leader Eric Cantor (Va.) rolled out last week -- which would allow the Senate to strip out the measure defunding Obamacare  and then send it to President Obama -- or take a hard line, making it so that Senate Majority Leader Harry Reid can still strip the defunding out the legislation but the measure has to then return to the House for passage. Boehner took the hard line, which tells you that he isn't willing to cross the cast-iron conservatives in his conference on this -- and that he likely won't change his tune by Sept. 30. (For further explanation on "cast iron conservatives", read this.)

2. Senate Minority Leader Mitch McConnell has no incentive to work with Reid (or any Democrat) to find a path to consensus before Sept. 30. In fact, McConnell has a huge disincentive to do so in the form of a conservative primary challenger named Matt Bevin. Bevin is already attacking McConnell as too much of an accommodationist and insufficiently conservative. And conservative groups like the Senate Conservatives Fund are essentially looking for a reason to throw their lot in with Bevin or, more accurately, against McConnell.  McConnell working with Reid (and/or Obama) would be treated as a provocation by these groups.  McConnell's reluctance to be involved is all the more important when you consider he was the critical Republican in cutting a fiscal cliff deal.

3. The calendar doesn't add up. Yes, Congress is like a college student -- staying up late to finish a paper (or a bill) the night before it's due.  We govern from crisis to crisis these days and, so far, Congress and the President have managed to turn in their paper just before they got an "F". But, Boehner's move today makes it almost impossible, in the most most literal, logistical terms possible -- to get something on President Obama's desk before Sept. 30. Under the most likely scenario in the Senate, the earliest that the House will get back a continuing resolution is Sept. 27 -- and that will be one that, unless something dramatic changes in the Senate, doesn't make mention of defunding Obamacare.

4. Democrats -- from the White House to the Congressional leadership -- won't blink. President Obama, Reid and House Minority Leader Nancy Pelosi (Calif.) are well aware of the public polling that shows Congressional Republicans would bear the brunt of the blame if the government shuts down. And, they are even more aware of the deep split between establishment Republicans and the cast-iron conservatives in the House. Add it up and there's no incentive for Democrats to throw Boehner a political lifeline on the government shutdown -- particularly given his decision to push forward with the defunding Obamacare effort. This is a you-made-your-bed-now-sleep-in-it moment for Boehner in the minds of Democratic leaders.

5. The cast-iron conservatives won't blink. If there is one single organizational principle that unites all of the cast-iron conservatives in the House and Senate, it's their vehement belief that the health care law is a massive mistake in public policy and has to be repealed for the good of the country. It's literally unfathomable that they will capitulate on Sept. 29 to pass a Senate-approved bill without the defund provision in it. That means that if Boehner and Cantor want to pass the CR, they will have to do so with a significant number of Democratic votes to make up for the losses they will suffer in their own ranks.  And, if Boehner does that, it could be very detrimental to his chances of being Speaker again in 2015.