After my swearing in as a new MP in 1997, I shook hands with the Speaker, Betty Boothroyd.  “What’s your majority?” she inquired.  “3,500”, I dutifully replied.  “You’ll increase it” she soothed back, as I was ushered on my way.

Now Betty was a formidable woman, but I doubted that she had such confident powers of prediction.  However, she was right.  Four years later, and my majority nearly doubled, despite the national Labour vote dropping and a small net swing back to the Conservatives.

What Betty knew about, and I didn’t, was the ‘incumbency bonus’ – a tendency, prevalent through most post-war elections, for newly elected MPs to do better in their first bid for re-election in their constituencies than their party would do nationally.  Thus incumbency factor helped give Tony Blair his second landslide, very nearly comparable to that of 1997, despite a drop in national support.

Now, with the election just months away, there are many Conservative strategists, and many prominent pollsters, arguing that the incumbency bonus will again come into play in 2015, giving the party a potential life-line in an otherwise close contest.

Based on recent elections, some argue that the incumbency bonus can be worth up to 3 percentage point of the vote.  Apply that evenly to all of David Cameron’s freshmen seeking re-election, and it puts enough of them across the winning post, in an otherwise, dead-heat  contest, to guarantee him leading the largest party.

So it’s no surprise that incumbency is thought of as a secret weapon quietly working to the Conservative’s advantage.  There is plenty of historical precedent for it, so no reason to suppose it won’t take effect again next May.

Only there is a problem with this belief.  There is a shortage of supporting evidence.  Instead, it looks as if the incumbency bonus may become a victim of the new game of pentagonal politics.

The evidence?  More so than in any previous election run-up, we are getting a decent crop of constituency specific polling, thanks to Lord Ashcroft.  This matters.  In a pentagonal contest, the national opinion polls matter for much less than before.  Around one-third of the vote is potentially going to parties other than Labour or Conservative.  The flow of votes directly between the two main parties is now negligible.  The larger movements are back and forth between either of the main parties and a variety of ‘minority’ parties.  This makes for much more marginality, and much greater scope for tactical voting.

We have Aschroft polls for 38 Conservative-held marginals with Labour as the main challenger.  The incumbency bonus is detectable in 12, non-existent in 10 and working in reverse effect in no fewer than 16.

If his findings hold good over all 90 Conservative seats gained by new MPs in 2010, the majority are not going to enjoy any incumbency bonus.  The secret weapon may either not go off, or may even blow up in the hands of its operator.

Why may the incumbency bonus no longer apply?   The answers lie in the nature of pentagonal politics:

Firstly, in Conservative-held marginal seats, with Labour as the close challenger, the signs are that the Liberal Democrat vote will drop dramatically, and the net movement will advantage Labour.

Secondly, in these seats, the UKIP presence, although greater than in 2010, will still be significant. None of these will be UKIP targets, but all the polling research to date indicates that UKIP supporters are pretty committed. Thus, even though UKIP’s share of the vote will not be large in these seats, it will still be a net loss for the Conservatives, as against Labour.

Finally, in these seats, with Labour as the clear challenger, there could be an increased willingness on the part of those who are anti-Conservative to vote tactically to remove the incumbent. Thus Labour’s potential loss of support to either the Greens or, to a lesser extent , UKIP, may be attenuated – and not as large as the Conservative’s net seepage to UKIP.

The Aschroft polling already shows that Labour is doing better in its target seats than is implied by its national poll ratings.  This probably reflects these factors, as well as targeted local campaign efforts.  Now it appears that these factors are also strong enough to act as a counterweight to the traditional incumbency bonus.

Was this, possibly, spotted by the eight Conservative first-termers in marginal seats who have already announced their intention not to stand next year?

Pentagonal politics may be about to consign the incumbency bonus to the history books.  The Speaker, when welcoming the new intake of MPs in May, will have to come up with some new words of encouragement.