There's no question House Democrats are determined to impeach President Trump. Some have been trying since shortly after his inauguration.

But a fundamental problem with the Democratic case against the president in the Ukraine matter is the same as the problem with their case against the president in the Russia matter. They are accusing Trump of attempted crimes that never actually came to fruition.

Of course, in the world of criminal law, attempted crimes are still crimes, albeit less serious than completed ones. But in the political process of impeachment, in which both sides are appealing to voters, Trump has a defense that will likely resonate with his supporters and some independents: The crimes Democrats have accused him of committing either never happened or were contemplated but never seen through to the finish. In addition, Trump himself does not view anything he did as criminal at all, hence his extraordinary openness with information, such as the rough transcript of his phone call with Ukrainian President Volodymyr Zelensky, plus his decision to allow White House staff to cooperate fully with special counsel Robert Mueller. Because of that, even if Democrats succeed in making a case against Trump, voters might reject removal from office, which is the political equivalent of capital punishment.

Three examples of such allegations against Trump:

1) Conspiracy/coordination. In the Russia affair, Democrats hoped to show that the Trump campaign conspired or coordinated with Russia to fix the 2016 election. Democratic lawmakers and their supporters in the press spent hour after hour predicting that Mueller would prove Trump-Russia conspiracy/coordination. But then, after two years of investigating, Mueller announced that he could not establish that any conspiracy or coordination took place. The prosecutor could not show that the alleged crime at the heart of the Trump-Russia affair ever actually occurred. Talk of impeaching the president for conspiracy/coordination soon disappeared.

2) Obstruction of justice. Also in the Russia affair, Democrats who were disappointed that Mueller could not establish conspiracy or collusion quickly pivoted to accusing the president of obstructing justice. On that topic, Mueller listed 10 episodes in which he strongly suggested obstruction took place, even though he reached no conclusion on any of them. But the obstruction charge also had a problem: Mueller's investigation was not actually obstructed by the president.

Democrats' Exhibit A, which, before the Ukraine story, was to be the centerpiece of impeachment, was the allegation that Trump ordered the firing of Mueller. The allegation rested mostly on the testimony of former White House counsel Don McGahn, who could not say specifically what the president told him but testified that Trump twice pressed him to call Deputy Attorney General Rod Rosenstein to have him fire Mueller. McGahn did not call Rosenstein, and Trump dropped it.

That meant the allegation upon which Democrats hoped to remove Trump from office was an order the president allegedly gave, which was not carried out, and which the president then abandoned. If it was attempted obstruction of justice, it wasn't a very determined attempt. Indeed, before the Ukraine affair burst into the news, Democrats were divided among themselves about whether to proceed with impeachment.

3) Quid pro quo. In the Ukraine matter, Democrats appear to be pressing the case that Trump "extorted" Zelensky. The idea is that Trump withheld U.S. aid to Ukraine until Zelensky began investigations into events during the 2016 election in Ukraine and the sketchy business dealings of Joe Biden's son Hunter Biden with the Ukrainian energy company Burisma. Zelensky was also to make a public announcement that the investigations were taking place.

Assume that was Trump's demand — the president denies it, but Democrats contend it is an open-and-shut case. There are still problems with the allegation. One, a number of people close to the matter say the Ukrainians did not know that Trump had placed a hold on the aid at the time Trump spoke to Zelensky in their now-notorious July 25 phone call. But even if the Ukrainians did know, it appears Zelensky did not do anything — no investigation, no announcement — and Trump gave up and sent the aid.

By the beginning of September, former U.S. Envoy to Ukraine Kurt Volker testified in the impeachment proceeding, "I think the Ukrainians felt like things [were] going the right direction, and they had not done anything on — they had not done anything on an investigation, they had not done anything on a statement, and things were ramping up in terms of their engagement with the administration." By law, the aid had to be given to Ukraine by Sept. 30, and it began flowing on Sept. 11. Nothing was held past any deadline.

There were similarities between the quid pro quo matter and the Trump-Mueller situation: Trump wanted something done. It wasn't done. He dropped it.

Now, after several days of still-secret testimony, Democrats say the quid pro quo has been definitively established. But it would be more accurate to say the attempted quid pro quo has been established. And even if that is true, not all of the attempted quid pro quo was improper. Why couldn't the president lean on Ukraine to investigate events related to the 2016 election? U.S. law enforcement has already done that with several other countries, and investigating the 2016 election has been going on for three years.

The attempted quid pro quo that, if proven, would damage Trump politically is the request for an investigation into the Bidens. Still, the bottom line is: 1) Trump withheld the aid, 2) he asked for an investigation and a statement, 3) the investigation and statement did not happen, 4) Trump released the aid. If that is what happened, will Republicans agree to remove the president on the basis of those facts? Will a majority of voters?

It's always wise to add a warning that there still might be shocking evidence that the public doesn't know that could change the situation. But right now, the Democratic case is perhaps not as strong as they would have you believe.