Washington (CNN) A push by House Democrats to impeach President Donald Trump by Christmas reflects urgent political pressures but also a deeper driving force: a belief that they have got the impeachment goods on him.

House Speaker Nancy Pelosi's generals are cranking up the pace of their investigation ahead of next week's televised hearings , which will stress the grave historic reality of what lies ahead.

It makes sense for Democrats to seize the moment as the attention of the nation is focused on impeachment , as a torrent of evidence runs in their favor and as flailing Republicans struggle to settle on a coherent defense of the President.

In a sign the White House is trying to stem the Democratic momentum, a White House official told CNN that acting White House chief of staff Mick Mulvaney is unlikely to appear to testify Friday despite the subpoena he was issued Thursday.

And a lawyer for the whistleblower whose complaint triggered the inquiry sent a letter to White House counsel Pat Cipollone warning of possible legal action if Trump does not "cease and desist" attacking his client and any "physical harm" comes to the whistleblower or their family.

Polls showing a modest spike in the number of Americans in favor of impeaching and removing Trump are another argument for a swift process before the public tires of the whole thing. Speed could help Democrats fashion a concise but direct argument that Trump abused his power by using his authority to set foreign policy in order to demand political favors of a vulnerable government in Ukraine.

All that explains why when former national security adviser John Bolton refused to testify before impeachment investigators Thursday, they declined to issue a subpoena even though he is considered an insider witness with a colorful story to tell.

The ensuing legal fight would have stretched at least into the early months of next year -- stealing Democratic momentum -- when, CNN reported, party officials are looking to conclude House impeachment business this year.

Pelosi's team also has an electoral imperative to move quickly, presuming, as seems almost certain, they plan to inflict on Trump the legacy stain of being only the third impeached president.

Hearings before and after Thanksgiving would allow time for drafting articles of impeachment and a full House vote before lawmakers go home for the holidays. The political heat would then shift to the Republican-led Senate at the start of the new year.

A Senate trial might wrap up by the end of January, in time for Democrats to focus the nation on their primary race to find a candidate to take on the President next November. That's presuming, of course, there's no sudden collapse in Trump's support among GOP senators, who are expected to vote to acquit him of impeachable offenses and to keep him in office.

New momentum

But it's not all about politics.

The inquiry has acquired its own natural momentum. Hundreds of pages of witness testimony appear to bolster the Democratic case that Trump pressured Ukraine to investigate his potential 2020 rival, Joe Biden, and a conspiracy theory that Ukraine, not Russia, interfered in the 2016 election.

A number of officials whose depositions were released this week also believe that the President withheld military aid to Ukraine to get what he wanted from his counterpart in Kiev.

In new testimony released Thursday, for instance, senior State Department official George Kent accused Trump's personal attorney Rudy Giuliani -- who ran an expansive back-door diplomatic track to Ukraine -- of promoting a "campaign of lies."

"I do not believe the US should ask other countries to engage in politically associated investigations and prosecutions," Kent said, according to the transcript of his deposition.

Such evidence is why Democrats believe they already have a strong case against the President.

"The defense dollars to dirt scheme has come more into focus, and we have not get seen an arrow going in any other direction than this was a shakedown led by the President of the United States," Democratic Rep. Eric Swalwell of California said on Thursday.

Waiting longer for testimony from the likes of Bolton might bolster their wider argument and bring crucial new details -- but it also may drain more political momentum than it's worth.

The Democratic indictment of Trump will be based on three evidential pillars. First, the rough transcript of a call between Trump and Ukrainian President Volodymyr Zelensky on July 25, in which the US leader asked for a "favor" and mentioned Biden's name. Then there is a stream of texts between senior officials apparently discussing whether Trump was offering a quid pro quo for Ukraine.

That is now backed up by testimony from a group of diplomatic and military officials and former officials that appears to corroborate the case that Giuliani acted for Trump in a scheme that was designed to promote Trump's political prospects, not the national interest in Ukraine.

"I think it is a very strong case. A case I would absolutely be confident with as a prosecutor or an investigator," CNN legal analyst Elie Honig said on CNN International.

A chance for Republicans to steal the initiative

The Democratic approach does come with risks.

The transition to televised hearings next week is intended to place before Americans a concise and compelling case that Trump abused his power in Ukraine.

But it also gives the President's Republican allies the chance to cause havoc in the hearings, to attack and seek to discredit witnesses, and to stage the kind of procedural brouhahas that may give watching Americans the impression that Congress is just up to its normal partisan tricks.

However, the GOP has yet to come up with a rock-solid defense of Trump that can keep pace with the torrent of evidence unearthed by impeachment investigators.

GOP claims that Democrats are running a secret process have lost some resonance since the transcripts started coming out and may seem even less potent amid open hearings.

The President is complaining that he is being treated unfairly, for instance in a tweet on Thursday.

"It was just explained to me that for next weeks Fake Hearing (trial) in the House, as they interview Never Trumpers and others, I get NO LAWYER & NO DUE PROCESS. It is a Pelosi, Schiff, Scam against the Republican Party and me. This Witch Hunt should not be allowed to proceed!" Trump wrote.

Trump's complaint, however, ignores that the House impeachment process is like the indicting stage of a criminal proceeding. He will have his chance to mount a full defense in a Senate trial.

Republicans will also have the right to cross-examine witnesses in the open hearings -- just as they did in the closed-doors deposition process. They still, however, will be hampered by the disadvantage of being in the governing minority.

Given the apparent strength of the impeachment case, the GOP has resorted to a parallel-reality defense.

Trump calls the call with Zelensky "perfect" even though it appears to include clear evidence of pressure on Ukraine and at the very least an implied quid pro quo.

Taking up a similar line, Vice President Mike Pence said Thursday, "The American people have the transcript of the President's call and they can see there was no quid pro quo. The President did nothing wrong."

Pence also made the somewhat-implausible claim that Trump was primarily interested in stamping out "corruption" in Ukraine -- a topic he has shown little interest in pursuing elsewhere.

Other Republicans have also twisted and turned on their defense of the President. Sen. Lindsey Graham of South Carolina said this week that he would refuse to read the transcripts because the probe was a bunch of "BS."

Another Republican, Sen. John Kennedy, resorted to personal abuse, blasting Pelosi as "dumb" at a rally in his home state of Louisiana on Wednesday night.

The fact that Republicans are preferring not to address the substance of the case against Trump must be encouraging to Democrats.

One viable counterargument would be for the GOP to concede that Trump's parallel foreign policy in Ukraine was unwise and perhaps unethical but does not reach the level for impeachment.

Such a conceit -- which some Democrats employed during Bill Clinton's impeachment -- is unlikely to sit well with Trump. who demands total loyalty and rarely admits any wrongdoing.

Creating pressure for such Republican splits is another reason for Democrats to seek to pass the baton to the Senate by the end of 2019. A trial in the other chamber, given political reality, is unlikely to result in Trump becoming the first president ousted from office by the Senate. But it could lead to some very uncomfortable months for the Republican Party.