The former Mid Ulster MP took what had traditionally been the second safest Ulster Unionist seat.
The Sinn Fein candidate insists ordinary unionists in Mid Ulster are increasingly using his party's advice centres.
Mr Donaghy said the problems relate back to the closure of the Whiteabbey and Mid Ulster hospitals.
In human terms you cannot get more highly charged a contest than the one taking place in Mid Ulster.
Mr McCrea was Mid Ulster's MP from 1983 to 1997, when he lost the seat to Sinn Fein's Martin McGuinness.
That's not to diminish the concerns of the electorate in Mid Ulster.
The two resignations follow the DUP and Ulster Unionists' decision to back an agreed candidate to contest the Mid Ulster by-election on 7 March.
The MP for Mid Ulster has said anyone passing by a house where a pipe bomb partially exploded on Saturday could have been seriously injured.
It appears southern areas and mid Ulster are the worst hit.
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Given that Mr McGuinness took more than half the Mid Ulster vote in 2010, his successor as Sinn Fein candidate, Francie Molloy, is the clear front-runner.
Dr McDonnell had had his chance to challenge her and had lost, would he now stand aside to let the Mid Ulster MLA try his hand?
Voting in Mid Ulster takes place between 07:00 GMT and 22:00 GMT on Thursday and we should know the winner in the early hours of Friday morning.
The pair resigned from the UUP earlier this month after the party agreed to a single unionist candidate to contest the Mid Ulster by-election on 7 March.
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Coincidentally, on the day the writ was moved for the Mid Ulster by-election, the Northern Ireland Office published its draft bill on double jobbing and other potential changes to the political system here.
But, in contrast to Eastleigh, where the future of the Westminster coalition may be influenced by the outcome, Mid Ulster appears likely to see the replacement of one abstentionist Sinn Fein MP with another.
It made me wonder whether, in covering the campaign for the by-election in Mid Ulster due one week later, a reporter could get away with a piece to camera explaining that "the stakes could hardly be any lower".
And following the decision of Martin McGuinness to resign as an MP in order to avoid so-called "double-jobbing", by holding a seat in both the House of Commons and the Northern Ireland Assembly, a by-election is also expected in his Mid Ulster constituency.
Mr Nesbitt said the UUP had "listened carefully to the many pro-union voters in Mid Ulster who were calling for co-operation in this election and for the opportunity to end the long drought that has seen Mid Ulster without representation in the House of Commons for 16 long years".
Ulster Unionist Party leader Mike Nesbitt MLA said they had "listened carefully to the many pro-Union voters in Mid Ulster who were calling for co-operation in this election and for the opportunity to end the long drought that has seen Mid Ulster without representation in the House of Commons for 16 long years".
Both men have quit the Ulster Unionist Party over the decision of the party to support a unionist unity candidate in the Mid-Ulster by-election.
She was well known for representing several leading nationalist causes in the mid-Ulster area.
Martin McGuinness has confirmed that he has formally resigned as the MP for Mid-Ulster.
Mid-Ulster DUP MLA, Ian McCrea, said the county Tyrone facility was one of the best within the company.
Mr McGuinness was elected MP for Mid-Ulster in 1997, but he has never taken his seat in Westminster.
Meanwhile, Mid-Ulster decathlete Paul Tohill bettered 7, 000 points for the first time at a meeting in California last week.
The Northern Trust said it would move complex surgery cases from Whiteabbey and Mid-Ulster hospitals to Antrim and Causeway hospitals.
They join current Environment Minister Alex Attwood and mid-Ulster MLA Patsy McGlone.
On Thursday, Basil McCrea and John McCalliser quit the UUP over its decision to support a unionist unity candidate in the Mid-Ulster by-election.
Mr McGuinness, who is also the deputy first minister of Northern Ireland, confirmed on Sunday that he had formally resigned as MP for Mid-Ulster.
"What we need to see now is the writ being moved for a by-election to allow the people of Mid-Ulster to have their say, " the spokesperson added.
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