Maybe Trump could be good for liberalism.
Not because liberals hate him so much that voters and donors are energized to elect Democratic candidates, though that's certainly possible.
But because for political liberalism — not in the classical sense, but in the modern American political sense — to work, voters need to believe that politicians are going to keep their promises. Cynicism is terrible for liberalism, because if voters discount the promises of liberal politicians, there's not much left that remains — other than maybe identity politics — to motivate voters or donors.
We've had a run of presidents recently who governed different from how they campaigned. Bill Clinton ran in 1992 promising a middle class tax cut and instead delivered a tax increase. George W. Bush ran promising a "humble" foreign policy and instead delivered the Iraq War. Barack Obama in 2004 made his national debut as a unifier: "there's not a liberal America and a conservative America there's the United States of America." But his presidency was marked by stark party-line partisan division over issues such as the health care law and the Supreme Court nomination of Merrick Garland.
Compare that with Trump. Love him or hate him, one has to concede that our president has, on a series of fronts, actually followed through on what he said he world do. On foreign policy, he scrapped the Iran deal and moved the American embassy in Israel to Jerusalem. On economic policy, he renegotiated NAFTA and cut taxes. On judges, he chose his Supreme Court justices from the list he had announced. On immigration, where I disagree with him, he's apparently following through on his promises to crack down. While Democrats and the press obsess over Trump's minor misstatements to depict him as a liar, on these big-picture issues he's shown himself to be more believable than most people, with some good reason, generally expect from politicians.
It's certainly possible that Trump is too polarizing a personality for him to get credit for keeping these promises. And it's certainly possible that if voters do give him credit, they'll see it as a way that Trump is different from career politicians, rather than generalizing it into a general principle that now we can again believe political promises. But there's also an opportunity, or a risk, depending on how you see it, that Trump's follow-through could redound to the benefit of more liberal politicians, who would then be more likely to be believed if they go around saying, for example, that they are going to raise taxes only on the "rich," and that the money will only be spent on education and transportation.