I don't think Abbot is a serious contender, but McDonnell is. He's got a lot of support in the trade unions and his stanch on Iraq and social issues would appeal to the general membership, but there's little support for him in the parliamentary party.
While it is a valid point that a leader needs to be be supported by the people he leads, McDonnell's woes highlight the shift in the Labour party away from its roots as a union and people's party to a parliamentary affair - the parliamentary party obviously think that they know best who should be allowed to contest the leadership.
A contest between those candidates will be one that isn't viewed as legitimate, as the membership won't be allowed to vote for candidates it wants to, but choose between the candidates the parliamentary party thinks it should be offered. So the new leader will not enjoy the support of the party as a whole, and the decline in membership and support will not stop.
McDonnell's colleagues in the parliamentary party don't seem to recognize the reason they are currently sitting on the opposition benches is, partly, attributable to the likes of Ed Balls and the Miliband brothers, who have already secured enough nominations, and Andy Burnham, who will probably get there - though as he brings nothing different to the contest, it won't be any loss if he doesn't.
It's also worth noting that the parliamentary part were hardly a model of loyalty or support to for Gordon Brown while he was prime minister, and this continual politicking and knavery contributed to the defeat. The problem, it seems, isn't so much the leader but the parliamentary party itself. it's stuffed with opportunistic careerists. McDonnell, at least, can not be accused of putting his career ahead of principles.
Asking people to choose between candidates associated with the blunders of the last government will leave party members feeling bitter and disillusioned, and with good reason. These people showed themselves to be wrong, unprincipled and unelectable. Why the fuck do we have to choose one of them to be leader, rather than someone who was actually right on key issues, isn't tainted by the last administration, and who opposed all the bad and stupid policies of the last administration?
The parliamentary party needs to stop trying to fix the election in favour of one of the failures from the last administration, and allow the membership to choose.
1 - Nominations tracker on the Labour Party website. The figures for numbers of nominations received as as of 7th of June, 2010. (http://www2.labour.org.uk/leadership-2010)