As President Trump passes the 100-day mark of his administration, House Republicans should be starting to worry about next year’s midterm elections.

The midterms are a referendum on the presidency, and preliminary signs point to problems for the party in power unless things change.

Trump’s approval ratings, which started low, haven’t budged. He’s in negative territory, with an approval rating of just 43 percent and a disapproval rating of 52 percent, according to the RealClearPolitics average. That’s a zone of negativity that, if history holds, could put the GOP’s current House majority in jeopardy in 2018.

Ronald Reagan, Bill Clinton and Barack Obama all had approval ratings in the low-to-mid 40s at the time of the first midterm elections of their presidencies. Their parties all suffered substantial losses. Reagan was at 42 percent in 1982, and Republicans lost 26 seats. Clinton was at 46 percent in 1994, and his party lost 54 seats and control of the House for the first time in four decades. Obama was at 45 percent, and Democrats lost 63 seats and, again, control of the House.

Trump got no honeymoon after the contentious 2016 election. He started his presidency less popular than any president in the history of polling. At the 100-day mark, his approval rating is lower than any past president since polling began. He has governed in a way that has cost him little-to-nothing among the voters who put him in office, but in ways that have not allowed him to expand his support or his appeal. Republican elected officials should hope that something changes.

What’s particularly worrisome for Republicans is another pattern about presidential approval: Most presidents who have governed in the modern era have seen their approval ratings slide between the 100-day mark and the subsequent midterm election 18 months later.

Former presidents George H.W. Bush and George W. Bush are exceptions to this pattern, aided by external events. The terrorist attacks of Sept. 11, 2001, boosted Bush 43 and Iraqi President Saddam Hussein’s invasion of Kuwait in August 1990 and the subsequent buildup ahead of the Persian Gulf War boosted Bush 41.

Trump’s troubles aside, House Republicans, themselves, have done little to demonstrate they are ready and able to be a successful governing party. The rolling mess with attempts to replace the Affordable Care Act has been more sitcom than serious governing. One attempt to get a vote on the bill failed badly. Last week, White House officials pushed and pushed for a vote ahead of the 100-day marker. House leaders, rightly wary of another failure in prime time, resisted.

[Republicans grapple with governing in the Trump era]

That situation could change: The House could find a compromise measure that brings enough consensus to pass a health-care bill. But the absence of an agreement after weeks of negotiations suggests that enough Republicans fear the political consequences of supporting a measure that likely will result in millions of fewer Americans having insurance and less coverage for some who do.

Nor are there other apparent easy victories ahead. Trump’s budget calls for substantial increases in defense spending, offset by big cuts in many domestic programs and parts of the State Department. The president likely will get his money for defense, but even many Republicans are rebelling at cutting some of those domestic programs.

View Graphic Tallying President Trump’s first 100 days

Then there is the big tax plan the president’s team presented last week.

Trump talked repeatedly about taxes during his campaign and twice put out proposals to cut corporate and individual rates. And his advisers have had months to prepare a detailed plan, along with the customary documentation that shows how the changes would impact individual families, whether the president’s or the average middle-class family.

But the plan presented consists of a one-page sheet of paper with broad outlines and equivocal promises. And while Trump’s team has pledged tax relief for middle- and lower-middle class families, the outlines suggest the wealthy will benefit more. The tax team still has considerable number-crunching and calibration to make good the promise.

And while the Republican Party has long stood for deficit reduction, the guidelines indicate that Trump’s proposals would result in another big imbalance. Months of work lie ahead with no guaranteed outcome.

This is perhaps a glass-half-empty look at the state of things for the Republicans, and it is by no means the only scenario that could unfold. The president and his party have months to overcome the limitations and setbacks of the first months of this year.

They could pass health-care revisions. They could pass a big tax bill. Their scorecard could look substantially better ahead of the midterms than it does now.

The president could also defy conventional metrics, as he did during the campaign. He is, after all, anything but a typical politician. On Election Day, 6 in 10 Americans said he was not qualified to be president. More than 6 in 10 said he did not have the temperament to serve as president. Yet enough of them voted for him to make him the winner over Hillary Clinton.

[The Democrats’ conundrum as they fight for rural House seats]

Working to the Republicans’ advantage are the structural realities in a divided America.

The Cook Political Report’s latest assessment of House races lists just six in the toss-up category, three Republican seats and three Democratic seats. Another 10 Republican-held seats are considered soft, along with seven Democratic-held seats. All that can change with events, but the starting point is a reminder of the degree to which competitive districts have continued to disappear.

Earlier this month, the Cook report’s David Wasserman and Ally Flinn produced the latest index of partisanship of all 435 congressional districts. This is a biennial report that dates back two decades. The most striking finding, well highlighted by others, is the staggering decline in swing districts. In 1997, there were 164 swing districts. In the new analysis, there are just 72.

The House is currently sorted in a deeply partisan way. Trump won 230 congressional districts last November, and Clinton won the other 205. Just 35 districts voted one way for the president and another way in the House race. Of those 35 districts, Republicans hold 23 seats that went for Clinton, and Democrats hold 12 that went for Trump.

The record low number of so-called crossover districts came after the 2012 election, when there were just 26. But a longer time horizon underscores the convergence of votes for the president and the House district by district. During the 1990s, there were more than 100 such districts that split the allegiance. That’s another reminder that all politics is more national, less local.

Democrats have been heartened by results in two special elections this year. They lost the race in the 4th Congressional District in Kansas, but only by six points in a district Trump had won by 27 points and Mitt Romney won by 26 points in 2012. In Georgia’s 6th District, Democrat Jon Ossoff came close to winning outright in the first round of voting in a district that Trump won by two points and Romney by 24 points. Still, Republican Karen Handel, who finished second in the first round, could win the runoff in June, leaving Democrats 0 for 2.

Democrats have done little affirmatively this year to help their cause. They are banking on Trump to keep their base energized. If he settles into his office and becomes more disciplined in style and conventional in policy, that could change the equation. For now, Republicans should assume they are tethered to the president’s political standing. They and he should act accordingly.