Julian Zelizer is a history and public affairs professor at Princeton University, editor of "The Presidency of Barack Obama: A First Historical Assessment" and co-host of the "Politics & Polls" podcast. Follow him on Twitter: @julianzelizer. The opinions expressed in this commentary are his own.

(CNN) When President Trump takes unimaginable steps like separating little children from their families to prove a point about how tough he is willing to be on the borders, there is a natural inclination for Democrats to predict that he finally took a step too far. He finally did something that would break the camel's back, he has given the nation a reason to believe he is not fit to serve.

But before Democrats assume that the crisis on the border will bring this President down, they might want to take a look at the recent poll numbers from Gallup . Despite everything that has happened over the last year and a half, the findings show that the President is not quite as weak as some of his opponents are hoping. Last week his job approval rating averaged 45%, the highest that it has been since his first week in office and, this week, slipped slightly to 41%.

Indeed, his job approval ratings have remained in the 40s since mid-April when unemployment rates plummeted to historically low levels . Just as notable is the political breakdown of his rating, with a 38% approval among Independents and 87% among Republicans. If one adds a grade curve to these numbers, it's clear that outside of the Democratic Party, his approval is holding steady.

His rating now is similar to Jimmy Carter (42%), Ronald Reagan (44%), Bill Clinton (44%) and Barack Obama (47%) in June of their second years. The good news for Democrats is that all four of those presidents experienced difficult midterm elections. In 1978, the conservative coalition of Republicans and Southern Democrats cut into the size of Carter's Democratic majority, with the emergence of new firebrands like Newt Gingrich of Georgia who were determined to cause the President problems.

Four years later, the Gipper lamented as House Democrats increased their majority by 26 seats , with most of the gains being liberal Democrats. And of course, Clinton saw Republicans regain control of Congress in 1994 and Obama watched as the GOP took over the House in 2010. None of this bodes well in 2018 for the Republicans, who are likely staring at a difficult midterm.

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