On each, Trump’s approval has declined. While his support among Democrats slightly increased (albeit from a very low base) on the economy and veterans, the biggest change on each issue — for the worse — has been among Republicans. Health care has been the biggest loser, where GOP approval of Trump’s ability to handle the issue fell from 81 percent in January to 65 percent in August.

Interestingly, we found that Trump’s approval has barely shifted on 11 other issues, including terrorism, gun control and education. Opinion has changed on the most visible issues of Trump’s presidency, which suggests that the public is reacting to the administration’s handling of specific policies. But the point is that Trump’s fellow partisans — the people we might expect to stick with him — have become more critical of the way he’s handled several key policy issues.

Parties can lose support when voters question their competence

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As it turns out, Trump’s experience is similar to what we’ve found in a recent cross-national study of public opinion toward political parties. Although partisanship is a powerful force that helps political leaders maintain the support of their base, partisans frequently update their perceptions of the competence of their party on issues. In our new book, we find numerous instances from the United States, United Kingdom, Australia, Canada and Germany in which voters lose faith in their own party’s competence on important issues.

We studied almost 40 cases in which parties saw significant changes over time in public ratings of their ability to handle issues. We found that these issue-handling reputations declined significantly in the wake of a symbolic policy shift, such as landmark legislation, or a major shock or failure, such as failed military campaigns or economic crisis.

When voters lose confidence in the competence of a party, the change tends to be long-lasting. And those changes are brought about by shift in opinions among a wide group of voters, not just a party’s political opponents. They cut through partisanship in a way that even a party’s supporters as well as independents share the same assessments of a party’s (or a president’s) relative strengths and weaknesses. Ultimately, we find that some demonstrations of incompetence are so obvious that even a party’s loyal supporters can’t deny them — for example, the Bush administration’s costly interventions in Iraq and Afghanistan resulted in a decline in the GOP’s traditionally strong ratings on defense.

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Declining competence ratings can make it hard for parties to win elections

Falling assessments of a party or president’s competence have important electoral consequences. For one thing, it limits the range of issues that a party can campaign on, since voters are unlikely to find persuasive promises from a party that has proven it can’t deliver. Voters punish governments for poor performance on a range of issues — not just on the economy, as some claim. It also can contribute to an accumulation of bad news stories over time, which can dog a party’s election efforts and lead to “costs of ruling” — the downward trend in support for incumbents over their time in office.

These are all reasons that Trump’s failure to deliver on his flagship promise to repeal and replace Obamacare and his declining ratings on key issues are troublesome for the GOP. Unless there is significant rebound in public opinion, it will be difficult for Republicans in 2018 — or Trump himself in 2020 — to run on a record of policy accomplishment. This may help to explain why Trump has in recent days demanded that Congress fund his signature campaign pledge, a wall on the Mexican border.

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Of course, competence assessments aren’t all that matters in elections. In our research, we found that when campaigns aren’t framed around a choice between a competent party and an incompetent one, then evaluations of issue-handling ability tend not to matter as much.

But if competence becomes the focus of attention and a president loses his reputation for being trustworthy, capable and effective, then the electoral consequences can be devastating. This is one reason that the decline in assessments of Trump’s issue-handling ability, especially among Republican voters, is so important. Even partisanship may not be enough to save him.