Analysis by Chris Cillizza, CNN Editor-at-large

(CNN) -- The news that Speaker Paul Ryan is engaged in "soul searching" about his political future and could leave Congress after the 2018 election shocked a political Washington sprinting toward a tax cut vote and Christmas recess.

It shouldn't.

While Ryan insists he isn't leaving anytime soon -- not a denial, it's worth noting! -- there's a very strong case to be made that quitting at the end of 2018 is the best possible move for his political future.

Here are five good reasons why.

1. The George Costanza Effect: There's an episode of "Seinfeld" in which George realizes that his best move at work is to make a joke that people laugh at and then leave -- he calls it going out on a high note. (See the technique here.) The theory is that you should leave on your own terms and at a moment when people have a good feeling about you.

Assuming the tax plan passes Congress sometime before Christmas, that may be as good as it gets for Ryan -- a nerd who allegedly always dreamed about chairing the tax-writing Ways and Means Committee. He can walk out of the Capitol with his head held high and his legacy intact: The man who stepped into the breach following John Boehner's surprise resignation and helped shepherd a major tax cut through Congress.

2. The 2018 midterms: As of right now, it's a 50-50-ish shot as to whether Ryan will even have a speakership to walk away from come late 2018. Results in Virginia and in the Alabama Senate race on Tuesday suggest a hugely engaged Democratic base and a far less enthusiastic Republican base. A new Monmouth University national poll gives Democrats a massive 15-point edge on the generic ballot question. In short: All signs point to a wave for Democrats next November.

Announcing a plan to head for the hills prior to the election insulates Ryan from the narrative that he was was forced to flee Washington with his tail between his legs after voters rejected his party at the ballot box.

3. Donald Trump: Yes, I know that people who are in a position to know say that Ryan and Trump get along better than was expected. But Ryan, more than anyone, has to know that Trump is mercurial and unpredictable. Who's to say that Trump's relationship with Ryan won't go the way of Trump's relationship with Senate Majority Leader Mitch McConnell -- where the President was openly suggesting that McConnell might need to step aside if tax reform didn't pass?

Dealing with Trump is a fraught endeavor for any politician, but especially so for someone like Ryan, who is in the bull's-eye if Trump is looking for a new scapegoat. Why hang around and wait for that anvil to drop on your head?

4. This isn't Paul Ryan's party anymore: Ryan soared to national prominence on the wings of his much-ballyhooed budget, which offered deep cuts to entitlement programs in order to reduce the national debt. Trump's takeover of the Republican Party during the 2016 campaign pushed all of that aside. Trump promised to preserve Social Security and Medicare in their current form. And Trump has shown little concern for growing government spending in order to get his priorities accomplished.

The priorities that Ryan cares most about, then -- aside from tax reform -- ain't happening in the current iteration of the Republican Party. And, if anything, the party's grassroots are moving closer to Trump's priorities -- economic nationalism, skepticism of trade deals, etc. -- rather than Ryan's. Why stick around to defend a series of principles and priorities you don't believe in?

5. The presidency: Ryan insists he has already had the job he always dreamed of: chairman of the Ways and Means Committee. Color me skeptical. That's like a basketball player saying he always dreamed of playing in the NBA developmental league. Politicians -- especially the ambitious ones, which is most of them -- aspire to one thing and one thing only: being president.

If Ryan does have some interest in running for president -- and he's got oodles of time, since he is only 47 years old -- getting out of Washington and the speakership is a very good idea. (I wrote when Ryan agreed to be speaker that it was a bad move for his presidential prospects.)

People hate Washington. Especially the Republican grassroots. (How do you think we got a President Trump?) The less associated Ryan is with the near-certain legislative failures to come over the next few years, the better for his chances of running for president successfully one day. Plus, if the Trump brand continues to struggle -- 32% approval in a recent poll -- then Ryan would also benefit from being several states away from the White House for the final two years of Trump's term.

All of the above is to say that if Ryan isn't seriously considering leaving following 2018, he should. For his own good.