Donald Trump speaks during the Values Voter Summit in Washington. (Jose Luis Magana/Associated Press)

Donald Trump is upset the media are reporting his slip in the polls. “I’m leading every poll nationwide and I’m leading every state,” he complained to a cheering crowd in Oklahoma this weekend. “The press and the distortion they write is unbelievable . . . They’re terrible people.”

Perhaps, but in this case they are accurate. Yes, Trump is still leading, but his decline is real. Before the CNN debate, he was in the low-30s in a series of polls (The Post/ABC News poll had Trump at 33 percent). Now, post-debate, Trump has dropped to the mid- to low-20s — and a new NBC News/Wall Street Journal poll Sunday shows him statistically tied, 21 percent to 20 percent, with Ben Carson.

Trump is far from finished, but he is at a crossroads. What helped rocket him to the front of the pack — a combination of refreshing political incorrectness and vague promises to “make America great again” — is no longer growing his support. Yes, his hard-core followers will not likely abandon him. But his ability to build beyond his base is limited by two problems: First, his name-calling act is getting old, and second, his campaign is a virtually substance-free zone.

Trump got booed this weekend when he called Marco Rubio a “clown” at the Values Voter Summit. His attack on Carly Fiorina’s looks (“Look at that face! Would anyone vote for that?”) also backfired. At first, it was exciting when Trump called people “stupid” and criticized his opponents’ looks and acumen. But the excitement has waned over time. Voters loved it the first time they heard New Jersey Gov. Chris Christie tell someone to “sit down and shut up” at a town hall meeting. But after a while, it just began to sound boorish. Trump is starting to sound that way, too.

It would be one thing if the barrage of insults was coming from a candidate in command of the issues — but the debates have exposed the fact that Trump does not possess a working knowledge of many the critical matters that will face the next president. This is especially true on foreign policy. During the CNN debate, Trump was asked about Russia’s recent troop deployments to Syria. His answer? “Syria’s a mess. You look at what’s going on with ISIS [the Islamic State] in there, now think of this: we’re fighting ISIS. ISIS wants to fight Syria. Why are we fighting ISIS in Syria? Let them fight each other and pick up the remnants.” (On “60 Minutes” this Sunday, he repeated this line and added, “Maybe let Russia do it. Let them get rid of ISIS. What the hell do we care?”).

Donald Trump heard boos at the annual Values Voter Summit for calling Sen. Marco Rubio (R-Fla.) a "clown." (Family Research Council)

This absurd response stood in stark contrast to the substantive answers that immediately followed from Rubio and Fiorina, who both showed mastery of the subject — and surged after the debate. Trump looked out of his depth — and lost support.

Trump deflects foreign policy questions by saying he does not want to tell us his plans because he does not want to tip his hand to Russian President Vladimir Putin and other world leaders. Voters see through this dodge. Ronald Reagan told us his plans; so can Donald Trump. It is becoming increasingly obvious that Trump won’t tell us what he plans to do because he does not know.

There are only four more debates before the first votes are cast in Iowa. Each successive debate hurts Trump’s standing, as his lack of policy depth is exposed. The irony that after Mitt Romney’s 2012 battering, Republican leaders limited the number of debates in order to protect the establishment front-runner. Instead, their rules are protecting Trump.

It is far too soon to write off Trump. He still has a lot of things working in his favor. He is still leading in all the key battleground states. While he may be leveling off, every Republican candidate would trade their poll position for his. Trump also has crossover appeal. His attacks on China and Mexico and opposition to free trade resonate with Reagan Democrats. He has charisma and a great slogan, “Make America Great Again.” If his goal were to sell baseball hats, that might be enough. But if he wants to be president, he needs to explain how he’s going to make America great again. Americans are angry at the political establishment, but they are not likely to elect someone on a platform of nothing more than “trust me.”

Even Trump seems to recognize this, which is why he is finally unveiling a tax plan Monday. Perhaps he will finally start offering details in other areas as well. Then we can have a debate among the candidates over substance rather than looks. This is clearly what voters want.

One woman who drove more than 2 ½ hours to his Oklahoma rally this weekend told The Post: “I want to hear what he has to say. I want to hear more than ‘Let’s make America great again’ and we’ll be good. I want to hear some issues.”

She didn’t get her wish. If Trump wants to win voters like her, and the presidency, he’ll need to start demonstrating some depth. Trump keeps on telling us how “smart” he is. Well, if he is really as smart as he says, he needs to stop telling us and start showing us.

Republican presidential candidate Donald Trump released a tax proposal, outlining “major” tax cuts while helping to “balance budgets.” Here’s exactly what’s inside Trump’s tax plan. (The Washington Post)

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