Well, it could be. David Mitchell has a nice piece in the Guardian which sums it up: usually a government this discredited would lose the election to the main opposition party, if they got their act together. The Conservatives took a long time to get their act together but finally settled on an articulate and telegenic leader just as Labour's political strategy started to fall apart. So, they should be winning. The latest polls from UK Polling Report give the Tories about 34-5%, Labour and Lib Dems around 28-30%, leaving the Conservatives 61 seats short of a majority (assuming uniform swing, which is the only assumption we have available).
This is not the first time the third party has made an impact - in 1983 the Liberal-SDP alliance got 26% and nearly pushed Labour into third place. But Thatcher's Conservatives won about 42%, and a big majority. This time, the third party is taking votes off both the 'major' parties. David Owen and the other SDP leaders talked of 'breaking the mould of British politics' - they did, but in parliamentary terms nobody noticed. But this time it's different, as they say. The Conservatives are penalized by the constituency boundaries we currently have, so they need way over 40% to be sure of a majority. They haven't had those kind of numbers since well before the campaign started.
Second, Cameron tried to make the Tories look more appealing, but they remain unappealing, so the only workable strategy is to pretend to be something they are not. Young, liberal, colour-blind, open-minded about sexual preference, concerned about the poor. It's just not credible - anybody with these views would not join the Conservative party.
So, rather than Tony Blair, Cameron is really Neil Kinnock. He's done the spadework to make the party electable, but doubts remain. But they probably won't get another chance. If the Conservatives don't win a majority, electoral reform will put one beyond them forever. I hope.