It would enhance his bargaining power, while the Tories can remind him that they allowed a vote on AV.
Were the opposition to be beaten by the Tories and Ukip, it would send shockwaves through the PLP.
The PM recognises how any hint that he could depart could galvanise the "No" campaign - but he is preparing for defeat.
A strong performance by Farage's party could signal to Scotland that England may vote to leave the EU in a future in/out referendum.
But Ukip is betting on a late surge.
The party's new campaign is designed to win votes by whipping up animosity against foreigners living and working and contributing to this country.
While the Tories, the Lib Dems and UKIP are silent, we are fighting for the health service to be protected from privatisation under the new EU-US trade treaty.
There are good grounds for believing we will succeed in ensuring the NHS is exempt from the transatlantic agreeement.
Eurosceptics harp on about the need for democracy. But the Swiss, like the Norwegians and the Icelanders, choose to eat food from a table at which they have no seat.
Conservative peer also says that Britain will join the euro, that UKIP is a "racist" party and that Boris Johnson shouldn't stand for parliament before 2016.