Donald Trump speaks during a thank you rally in Ladd-Peebles Stadium on December 17, 2016 in Mobile, Alabama.

Ex-Sen. Tom Coburn has offered a theory for why so many Republicans still back President Donald Trump despite behavior that has alienated so many others.

"We have a leader who has a personality disorder," the Oklahoma Republican told The New York Times last week, but "he's done what he actually told the people he was going to do and they're not going to abandon him."

The first assertion offered a damning description of the president. But the second, in a different way, damns his party.

That's because Trump's aides and allies in Congress have spent much of this year trying to keep him from doing things he said he'd do in last year's campaign — which, in turn, shows how Republican politics have drifted from governing realities in 21st century America.

The delay in the tax bill Trump and GOP leaders had hoped to release on Wednesday provides an example. Candidate Trump initially proposed a cut so massive it would have taken millions more Americans off the tax rolls, cut the top personal rate to 25 percent and cut the top corporate rate to 15 percent.

Trump claimed it would produce such an explosion of growth that the deficit wouldn't grow at all. But the conservative Tax Foundation forecast $10 trillion in higher deficits, and the plan has been whittled down since.

Congressional Republicans acknowledge the scaled-back version — with a top corporate rate of 20 percent and top personal rate of at least 35 percent — will increase the deficit by $1.5 trillion. Red ink would rise even more unless the plan includes offsetting provisions to raise revenue.

But Trump has opposed some prominent options. And the resulting difficulty in making the plan add up contributed to the delay.

The same problem hobbled the GOP effort to repeal and replace Obamacare. Trump pledged better, cheaper health care for all Americans without touching Medicaid — a standard that Republican plans could not meet.

On other issues, advisers have kept Trump from labeling China a "currency manipulator" and imposing retribution on American companies that move factories offshore.