218, 217, 216…. It’s a familiar benchmark: with 435 members of the House, legislation needs a minimum of 218 votes. But a vote on health care reform will probably end up needing a minimum of 216 votes. It’s worth taking a moment to explain why.

The issue is vacancies. Robert Wexler (D-Fla.) retired from Congress last year, bringing the total number of House members to 434. Last month, John Murtha (D-Pa.) passed away, bringing the total to 433.

Yesterday, there was another departure

Hawaii has an opening in the U.S. House of Representatives after now-former Rep. Neil Abercrombie [D] resigned yesterday to run for governor.

…with yet another on the way.

U.S. Rep. Nathan Deal (R-Ga.) said Monday morning he will resign from Congress to “devote my full energies to the campaign for governor.”

Deal is one of seven Republicans seeking the GOP nomination for governor. He will resign from the U.S. House on March 8.

So, next week, the House will have 431 members, which means the threshold for a majority will be 216 votes.

As this relates to health care, it makes passage ever-so-slightly easier. The legislation passed in November with 220 votes, and when it comes time to take the next step, Speaker Pelosi will need four fewer votes to succeed.

The bad news is Pelosi and the Democratic leadership have lost four reform supporters (Wexler, Murtha, Abercrombie, and Joseph Cao, who has said he’s changed this mind). The good news is, the margin for error is a little more favorable with the lower “magic number” for success.

To reemphasize, this won’t be easy. Several Dems who voted for the bill in November — it’s hard to say exactly how many — did so because it included the Stupak amendment. If/when the House takes up the Senate bill, an unknown number of these Dems will likely withhold support, meaning Pelosi will have to make up ground elsewhere. Getting Blue Dogs to switch from “nay” to “aye” is no easy task.

But Deal’s departure gives the Democrats a bit of a hand, and given the challenge, every little bit helps.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.