President Trump hit the campaign trail Tuesday riding high with his best poll numbers since his inauguration — or maybe not.

His approval ratings range from the high 30s to the mid-40s, so the true measure of Mr. Trump’s support is anybody’s guess.

Mr. Trump scored his all-time high of 45 percent approval in a recent Wall Street Journal/NBC News poll, but his numbers fell in other surveys. A Quinnipiac University poll showed his 38 percent approval rating was down 5 points from June.

The president, whose supporters famously defied polls and pundits in 2016, continues to be a nightmare for pollsters. Once again, he is confounding anyone trying to gauge whether he will be a boon or a bust for Republicans in this year’s congressional elections.

The Rasmussen Reports daily tracking poll Tuesday gave him a 46 percent approval rating, back where it was a week ago amid fierce criticism of Mr. Trump’s kid-glove treatment of Russian President Vladimir Putin at their meeting in Helsinki.

Gallup pegged Mr. Trump’s approval at 42 percent in a polling average for the week of July 16-22, a 1 percent decrease from the previous week.

Predictions are difficult with Mr. Trump because the usual rules of politics don’t apply, said Vanderbilt University professor Marc J. Hetherington, who specializes in party polarization and voter behavior.

“On the one hand, the constant scandals capped by his performance in Helsinki might have ended other presidencies. On the other, the kind of success that the economy has enjoyed under his presidency would ordinarily produce approval ratings much higher than he has. The bottom line is that he’s the most polarizing president since we’ve had public opinion polls,” he said.

The professor added, “Unless something new and even more dramatic happens between now and November, it feels to me that the president is something of a wash in the midterm races.”

Others saw Mr. Trump tilting the scales toward Democrats in the same way his job performance ratings tilt toward disapproval.

“I wouldn’t say his job approval is up, but I would say it’s stable,” said Ron Faucheux, who runs the nonpartisan polling firm Clarus Research Group in Washington.

A polling average used by Mr. Faucheux has consistently put Mr. Trump’s approval rating in the 43 percent to 45 percent range.

As long as Mr. Trump’s negative rating tops 50 percent, he said, it is likely he will help Democrats in blue states, blue districts and in most swing states.

“Trump won 46 percent of the vote in 2016, so his current job ratings are a little under that point. If he can keep his ratings at or near that point, it should be enough to keep Republican candidates afloat in most red states and districts,” said Mr. Faucheux.

Mr. Trump remains in demand on the stump for Republican candidates, especially in red-state Senate races where Democrats are on the ropes.

Missouri Attorney General Josh Hawley, the Republican challenger to Democratic Sen. Claire McCaskill in November, got a boost Tuesday from Mr. Trump.

At a Veterans of Foreign Wars convention in Kansas City, Mr. Trump hammered Ms. McCaskill for her “tremendous opposition” to the tax cuts that passed late last year.

“She voted against. Unbelievable. And she wants to now end it so that you pay more. You figure this one out. I don’t know, is that good?” said Mr. Trump.

Mr. Trump won Missouri by more than an 18-point margin. He also has a strong base in states including Indiana, North Dakota, Montana and West Virginia, where Democratic senators are endangered this year.

The president regularly points out that no Democrats in Congress voted for the tax cuts, which he credits with spurring an economic surge that has driven down the unemployment rate.

It’s a message that resonates with the Trump base and could give some Democratic voters second thoughts about their party. But the tax cut message will lose appeal if the economy falters before November.

Mr. Trump’s best poll numbers are for his handling of the economy. The Wall Street Journal/NBC News poll found 50 percent approval on the economic front, 41 percent for the border and immigration and 26 percent for U.S. relations with Russia.

“The magnitude to which Trump hurts or helps all depends on what kind of district you’re in but, overall, Republicans seem more wary of alienating their base voters who are very supportive of Trump, versus confronting the president as a demonstration of independence,” said Republican Party strategist Kevin Madden, who was a top aide to 2012 Republican presidential nominee Mitt Romney.

“The big risk for Trump right now is that he’s fomenting a trade war whose negative impact could be felt in local and regional economies right around the first week of November,” he said. “That’s bad timing for many Republicans who were hoping economic enthusiasm would be enough to buoy the president’s popularity and favorability ratings, which has a big impact in midterms.”

In Kansas City, Mr. Trump told the crowd that he is keeping his campaign promise to stand up to trade cheaters such as China and the European Union.

“They don’t want to have those tariffs put on them. They’re all coming to see us. And the farmers will be the biggest beneficiary. Watch,” he said. “We’re opening up markets. You watch what’s going to happen. Just be a little patient.”

The administration announced a $12 billion emergency aid package for farmers who have been targeted by China with retaliatory tariffs.

Trade war fears so far have not undercut Mr. Trump’s popularity.

His average job approval score in the Gallup poll inched up to 41.9 percent in the sixth quarter in office.

That is a personal best for Mr. Trump, though he still fares worse than every other president elected since World War II except Jimmy Carter.

Mr. Trump’s 41.9 percent average bested Mr. Carter’s 41.8 percent by one-tenth of a percentage point.

As in most polls, Mr. Trump’s numbers are bolstered by Republicans, who gave him an 88 percent average approval rating.

The average approval among independents was 36 percent. Among Democrats, it was 9 percent.

The approval rating among all party groups edged up in the sixth quarter: Republicans by 3 points, independents by 2 points and Democrats by 1 point.

“Trump’s job approval has not followed the normal trajectory for presidential approval, and that is one reason for the rare increase in approval at this stage of his presidency,” said Gallup pollsters. “Whereas most prior presidents started with high approval ratings at the beginning of their terms and saw those decline through the second year in office, Trump’s started with low approval ratings, but they have been better more recently, likely because of consistent positive economic news.”

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