To judge by Donald Trump's campaign rallies, the people in the crowd are still vocal and loyal, even as the President's troubles grow deeper.

He still seems to have retained his base, no matter how erratic his behaviour becomes, how many insults he hurls and questionable foreign policy actions he takes.

But beyond the true believers, the support that Trump needs most in Washington as he faces impeachment hearings is beginning to fragment.

Trump desperately needs to hold on to the loyalty of two groups of people: his aides and Republican senators.

And this is where he may most be in trouble — not with the voters who helped him win office.

Because Democrats control the House of Representatives, that first step in the two-step process provided by the Constitution to remove a president seems increasingly likely.

The remaining questions seem to be how the hearings will be conducted, what kind of evidence might be revealed, and the specific charges that may be levied against him.

The opportunities for evidence and revelations seem ample. The Trump administration has been rife with turnover, unlike any in modern American politics.

About 70 senior administration officials and cabinet members have quit or been fired by Trump since he took office in late January 2017.

And the resignations keep happening.

Trump's value as a job reference diminishing

On Friday, Trump lost his acting secretary of homeland security — a position that has had four different officials in charge during his tenure.

Others, who still hold their positions, are beginning to tell Congress what they know, and the longer they stay in this administration, the more damaging the information may be to the President.

Last week, Marie Yovanovitch, who was removed by Trump as ambassador to Ukraine, testified in private, despite efforts by the State Department to block her from doing so.

This week, Gordon Sondland, the US ambassador to the European Union, also plans to testify about the Ukraine matter.

This is somewhat of a surprise, given that Sondland has been seen as a Trump loyalist.

One of their colleagues, former US special representative Kurt Volker, quit his post, and then testified before Congress, illustrating the threat that Trump faces when his former allies turn on him.

Now that they are away from him, many former administration officials have had time to reflect on what is more important: their personal integrity, and/or their future ability to find credible employment, or their loyalty to an embattled president whose value as a job reference is diminishing by the day.

That is also a consideration for Republican senators, who will act as the vanguard against Trump's conviction by the US Senate, the second step in the removal process.

Just as conventional wisdom says Trump is likely to be impeached, so, too, do Washington experts believe Senate Republicans will keep him in office.

But things can always change, as they did for Richard Nixon in 1974.

Although the House held impeachment hearings, it never voted on whether to censure him, in part because he resigned before a vote could be taken.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 21 seconds 21 s Marie Yovanovitch testified in private despite efforts by the State Department to block her from doing so.

A key factor in his resignation came when Republican Senator Robert Griffin of Michigan travelled to the White House to tell Nixon he would be best off leaving Washington, since he was losing GOP support. (My late father, a staunch Republican, raised funds for Griffin's campaigns.)

Their defence of Trump and their votes in any Senate trial will undoubtedly be used against them in future campaigns.

Impeachment is becoming its own deadline

As yet, Trump has avoided an all-out rebellion by Senate Republicans, and if an impeachment case against him is deemed thin, one may not take place.

But little cracks are appearing in the Trumpian foundation.

South Carolina Republican Senator Lindsey Graham, a tireless defender of Trump since he took office in 2017, was infuriated by his decision to pull American personnel from Syria.

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Senate Majority Leader Mitch McConnell, the Kentucky Republican, has said he is duty bound to hold a Senate trial if Trump is impeached by the House.

That contrasts to his refusal to let the Senate consider the Supreme Court nomination of Merrick Garland, a judge that Barack Obama wanted to receive a court seat.

Meanwhile, some avowed Trump allies are declining to seek re-election, stealthily backing away from the chaos that he has created.

Six Texas Republicans have announced they will not run again, in what's been nicknamed the "Texodus".

To be sure, there are multiple reasons besides Trump that would cause them to leave.

Texas has one of the earliest deadlines in the country for entering races, meaning candidates must make up their minds by December 9 whether they want to be listed on its ballots.

In a way, impeachment is fast becoming its own deadline.

White House associates and Congressional allies need to decide by then whether they stand with Trump, or whether they want to distance themselves.

If they stay, Trump will become like the barnacles that stick to the underside of a ship — hard to remove without hauling a craft out of the water, and likely to weigh down the vessel, if it can make any progress at all.

Shaking free from Trump may not be simple.

Trump desperately needs to hold on to the loyalty of two groups of people: his aides, and Republican senators. ( Reuters: Jonathan Ernst )

Not only will the taint of their association with him be indelible, they are likely to incur his wrath and the possible retribution from zealots who remain in his camp.

On the other hand, Trump views his battle as his alone, anyway.

This weekend, he declared he was an "island of one", according to the Associated Press.

He was speaking of his actions on Syria.

But as impeachment proceedings move forward, that declaration could prove prophetic.

Micheline Maynard is an American author and journalist.