If we don't impeach Trump, that would effectively give permission to all future presidents to act as corruptly as they want—our system of checks and balances will fail. The president is not above the law. To support their case for impeachment, the House Judiciary Committee brought in four constitutional experts to clarify the case and rules for impeachment.
According to the New York Times, Harvard professor Noah Feldman argued Trump's actions rise to the level of impeachment for abuse of power:
“President Trump has committed impeachable high crimes and misdemeanors by corruptly abusing the office of the presidency,” Mr. Feldman said. “Specifically, President Trump abused his office by corruptly soliciting President Volodymyr Zelensky to announce investigations of his political rivals in order to gain personal advantage, including in the 2020 presidential election.”
Stanford law professor Pamela S. Karlan argued what Trump did is not politics as it's normally done—he was acting on his own behalf, not the country's.
“It is, instead, a cardinal reason why the Constitution contains an impeachment power,” she said. “Put simply, a candidate for president should resist foreign interference in our elections, not demand it. If we are to keep faith with the Constitution and our Republic, President Trump must be held to account.”
The Republican case for President Trump is that nothing actually happened. The money was released. The Ukrainians didn't feel pressured or threatened. Trump was fighting corruption in Ukraine—relying on a debunked conspiracy theory that Ukraine was behind the Democratic hack in 2016.
Trump's supporters say Democrats have no evidence because there was no crime. Again, nothing happened. Instead, impeachment is another example of how Democrats unfairly obstruct President Trump.
Mark Vespa, writing for Town Hall, says this is what actually happened:
In all, it was a series of career bureaucrats who disagreed with Trump on Ukraine policy. That’s not a crime. This being framed as the crime of the century is absurd. The Democrats needed smoking guns in September. And now, with no evidence, no movement in the polls regarding support for impeachment—the Left decided to forgo evidence in order to get Trump. That’s fine. They only look like bigger clowns and battleground state voters, who already are sour on this whole fiasco, will only be less inclined to vote Democratic in 2020. The Trump agenda has helped these states. They’re the ones who will decide the next election—and they’re not all that impressed with the 2020 Democratic field. And just like in 2016, a lot of these folks are former Obama voters.