By James Coomarasamy
BBC News, Washington
It was beginning to look rather awkward for President Obama.
Mr Daschle (L) has been a mentor to Mr Obama
To have one high profile nominee - Treasury Secretary, Timothy Geithner - reveal a tax problem was bad enough.
But when it emerged last Friday that his Health and Human Services Secretary-designate, Tom Daschle, had failed to pay around $130,000 (£90,000) in taxes on a car and chauffeur he had been provided by a business associate, a pattern of careless vetting seemed to be emerging.
And Tom Daschle was not just any nominee.
The popular former Senate Majority Leader was, in many ways, Barack Obama's Washington mentor.
It was Mr Daschle who persuaded the young man (who began his US Senate career just as Mr Daschle was ending his) not to delay his run for the White House.
And it was Mr Daschle who provided him with key inside knowledge and key members of staff - especially in the early caucus state of Iowa - to help him get there.
This closeness explains, perhaps, why the initial reaction of the President and Senate Democrats was to stand by their man.
On Monday, President Obama said he "absolutely" backed Mr Daschle.
The significance of Tuesday's decision may recede with time, but it was undoubtedly Barack Obama's worst day in office
The assumption was that - in the same way as Treasury Secretary Geithner - there would be an enquiry, an apology, but that the nomination would eventually be approved by the Senate.
It certainly looked that way after a chastened-looking Mr Daschle emerged on Monday evening from a meeting with his former Senate colleagues, who then queued up to play down his transgression and pronounce him the best man for the job.
But something changed overnight.
Just two weeks into the Obama presidency, a perception was beginning to form of a growing contradiction between the President's high-minded rhetoric on ethics and lobbying and the reality of his choices.
Having promised a new era of responsibility and trumpeted tough, new lobbying restrictions for his administration, exceptions were beginning to be the rule.
Aside from Timothy Geithner, there was an ethics waiver for William Lynn, the former lobbyist he had chosen to be Deputy Secretary of Defence.
And, as the media began to dig into Tom Daschle's own business dealings, an image was forming of someone who, while not a registered lobbyist, had inhabited a very lucrative grey area since leaving Congress.
The context was very different, but - in terms of perception, at least - there was an uncomfortable parallel between Wall Street bosses awarding huge bonuses - something dismissed by the President as "shameful" - and his own former political club of Senators closing ranks around one of their own; a man who had made millions of dollars, thanks to his political contacts.
It was starting to look not just careless, but hypocritical.
In an editorial calling for Mr Daschle to withdraw his candidacy, the New York Times referred to him as one of a long line of politicians who move "cosily between government and industry".
A president who had promised to bring change to Washington appeared to be endorsing what he repeatedly called on the campaign trail "business as usual".
And then came Nancy Killefer.
Her surprise withdrawal from consideration for the post of the government's Chief Performance Officer, because of her own tax transgressions, may have been the final straw; for the White House or Mr Daschle, it is not yet clear.
Whatever the case, just a few hours later, Tom Daschle surprised friends and foes alike by stepping down, even though only one Senator - Republican Jim DeMint - had publicly urged him to do so.
Mr Obama said he had accepted the decision with "sadness and regret".
And whatever relief there may be in the White House that a scandal has been nipped in the bud, Tom Daschle's loss is a big blow to the President, both personally and politically.
It is also a blow to advocates of health care reform - Mr Daschle's replacement is unlikely to have the same level of access to the Oval Office as he would have done.
The significance of Tuesday's decision may recede with time, but it was undoubtedly Barack Obama's worst day in office.
One that looks like the end of an initial honeymoon period for a President who has raised expectations to a level that is already proving hard to meet.