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President Trump raised eyebrows when he invited Democratic Sen. Heidi Heitkamp to fly with him aboard Air Force One for a tax-reform rally in her home state of North Dakota earlier this year. For a vulnerable Democrat running for reelection in a deep-red state that Trump won by 36 points, appearing with the president was a political gift. Trump called Heitkamp up on stage, shook her hand and heaped praise on her, describing her as a "good woman" — the perfect visuals for campaign ads portraying her as a moderate willing to defy the "Resistance" and work with the president.

The move puzzled Republicans, who wondered why Trump was giving a boost to one of their principal targets in the 2018 midterm elections. But then came the moment that Heitkamp must now regret. As he made his case for the tax bill, Trump turned to her and said, "Are you listening, Heidi?" And then he added this blunt message: "Do your job to deliver for America or find a new job."

Heitkamp did not deliver, nor did the four other Senate Democrats running for reelection in states Trump won by double digits: Joe Manchin III of West Virginia (a state Trump won by 42 points), Jon Tester of Montana (by 20), Claire McCaskill of Missouri (by 19) and Joe Donnelly of Indiana (by 19). Now it's time for Trump to make good on that threat.

With Trump's national approval rating averaging at just 38.5 percent — among the worst of any president in the first year after his election — some might suggest that he is in no position to impose political costs on his opponents. But his approval rating is between 50 and 60 percent in the five states where these vulnerable Democrats are running (except in Indiana, where it is 41 percent), making him a formidable adversary.

Trump should spend the coming weeks and months holding nonstop rallies in every one of these states to promote his tax reforms and how they will benefit ordinary Americans. He should tour companies that are using their savings from the corporate tax cuts to hire more workers, and businesses who are investing in new plants and equipment because the tax bill now allows them to write off those investments. He should visit small businesses who will benefit from the lower pass-through tax rate, so they can explain what it will mean to their workers. And he should hold town halls with middle-income families who will benefit from the individual rate cuts in the bill, so they can share what an extra thousand dollars in their pockets every year will mean to their families.

In each case, he should demand to know why their senator voted against the bill that made these things possible.

Heitkamp explained in a statement that she voted against the tax bill because, among other complaints, "middle income families lose out in the long-term because the relatively modest tax cuts they initially receive will expire after 2025." This is patently dishonest. Everyone knows the tax cuts for middle-income families won't ever expire. Even if Democrats are in power eight years from now, there is no way they won't extend the Trump tax cuts for middle-class Americans, just as Barack Obama extended all the George W. Bush tax cuts except for those at the very top.

According to the Tax Policy Center, 91.3 percent of Americans making between $48,000 and $86,100 will get an average tax cut of $1,090 next year. By the time the tax cuts require an extension, most will have saved nearly $9,000 cumulatively on their tax bills. That's real money. For those in the next income bracket — households making $86,100 to $149,400 — 92.5 percent will receive an average tax cut of $2,070. That's more than $16,500 in tax savings before the cuts are set to expire. Are Democrats really going to take that money away?

It was a big mistake for these vulnerable Democrats to vote against tax reform. They had an opportunity to work with the president and make the effort bipartisan — but to do so they had to go against their party. Instead, they cast their lots with the anti-Trump Resistance. Before the tax vote, Manchin dismissed from the pressure his constituents to support the president. "I just don't give a s---," he said, "Don't care if I get elected, don't care if I get defeated, how about that?" If he does not care whether he returns to Washington, then come next year the 68 percent of West Virginia voters who supported Trump may give him his wish.

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