The political landscape of an election takes shape long before any votes are cast, and judging by who is not running in 2018, the year is not shaping up well for the GOP.

This week, three more Republicans announced they would not seek reelection in the House of Representatives, bringing the number to 30 so far. That’s the highest number of retirements for a party in power for decades, surpassing the numbers in the wave election years of 2010, 2006 and 1994.

And a close look at those 30 districts suggests the GOP may have a hard time defending a big chunk of them. Past votes, analyst rankings, and demographics show the challenges Republicans may face in holding the seats – and consequently their current 46-seat majority in House.

It’s hard to overstate the importance of having a good, or even not-so-good, incumbent sitting in a congressional seat. House members are commonly reelected at a greater-than-90-percent clip and even wave elections don’t have an enormous impact. In the great Republican wave of 2010, for instance, the reelection number for sitting House members plummeted all the way to … 85 percent.

In 2010, 36 percent of open seats changed parties. In other words, it’s almost always easier to flip an open seat, particularly in a wave year.

So retirements are never a good thing for the party that holds the reins, but this group in 2018 may hold particular concern for Republicans. Consider some of the key data points for the 30 districts in question.

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Among the 30 retirement districts, there are five that Clinton won and three more that she lost by fewer than eight points in the last presidential race. In each one of those districts, the winning Republican candidate captured 60 percent or less of the vote in 2016.

The 2017 special elections showed one strong trend: both Democratic turnout and votes over-performed what they did in 2016. If that trend carries through to November, the Clinton vote in these districts could become a bigger share of the total tally – particularly without the familiar incumbent’s name on the ballot.

The analysts at the Cook Political Report rate 11 of the 30 Republican retirement districts as competitive – and for the most part, very competitive. (That includes all of the eight districts where Clinton won or was close.)

Cook says two of the 11 districts already “lean Democratic” while it rates five as true toss-ups. Among the other four, Cook rates two districts “lean Republican” and two are “likely Republican,” the safest competitive category for the GOP.

If the 2018 midterms become the wave that many expect, most or all of those seats are likely to fall into the Democratic column.

On top of those political measures of competitiveness, the 30 retirement districts show some demographic points of concern for the GOP.

Among the 30 retirement districts, 11 are above the national average for populations with a college degree.

That could be a problem for the Republicans. In the December NBC News/Wall Street Journal poll, 57 percent of those with a college-education said they would prefer that the Democrats control Congress, compared to 36 percent of the group who favor Republican control.

And the racial composition of the retirement districts should be worrying for Republicans as well.

Twelve of the retirement districts are above the national average, 38.7 percent, for their minority population. The Republican Party has long struggled to reach minority voters, and those problems have been exacerbated under President Donald Trump.

We have written a lot this year about the Republicans’ troubles in suburban areas, particularly the impact they had in the 2017 Virginia governor’s race. Wealthy, educated semi-urban areas seem to have serious issues with the Trump-era Republican Party and those issues could deeply damage Republicans in the retirement districts.

In half of the 30 districts, more than two-thirds of the population lives in densely populated urban/suburban areas. Some of the metro areas covered in the districts: Miami, Los Angeles, Houston, Detroit and Seattle. All hold cities that Democrat Hillary Clinton won by large margins in 2016.

Of course, none of this is a fait accompli. Republicans will rightly note that this is a very early look at the battlefield.

But so far, the poll numbers from early 2018 look a lot like those from late 2017. The trends that have been driving politics for the last year seem to be carrying over into the current climate, with President Trump at the center and most voters already decided on him.

In some ways the fact that “it’s early” makes the retirement numbers more troubling for the party. There are still a few weeks left in January, the prime time for retirement announcements. The coming days and weeks could bring news of even more Republican retirements in the House. And as bad as these numbers look, they could look worse by spring.