Lord Ashcroft, the Conservative Party's deputy chairman, is feared and loathed in equal measure by some of his Tory colleagues as well as political opponents.

To Labour and the Liberal Democrats, he is a bogey man who will not disclose his tax status and whose millions could swing the next general election through donations to Tory candidates in marginal seats. Many sitting MPs have the jitters about the likely impact of the highly professional, personalised mailshots already being sent to voters about the issues they care most about. They do not have the money to compete on equal terms.

Tory insiders insist that the party's opponents overstate Lord Ashcroft's power, but are happy for his spectre to give them the shivers. Internal critics claim he runs a secretive operation from his powerbase at the Conservative Campaign Headquarters at Millbank, Westminster, with more staff working directly to him than to the party chairman Eric Pickles. Critics wonder how the Ashcroft operation decides which candidates get what level of financial support.

Allies say his team is now fully integrated into the Tory machine and not the "party within a party" it was accused of being when Michael Howard, David Cameron's predecessor, kept Lord Ashcroft at arm's length. They say that Tory candidates are supported on the merits of their performance, not their political views.

No one doubts Lord Ashcroft's influence since Mr Cameron brought him formally on board. Insiders say his input on strategy is much more important than his money. They point to his candid pamphlet "Smell the Coffee; a Wake-Up Call for the Conservative Party", written after the 2005 election defeat, which urged the Tories not to repeat the mistake of preaching to its core vote on issues such as immigration. He warned that the Tories risked becoming "a rump" unless they reached out to professionals, women and aspirational voters.

"The Conservative Party's problem is its brand," Lord Ashcroft argued. The Cameroons agreed. "We are pretty much following the Ashcroft strategy," one senior Tory said last night.

Ominously for Labour and the Lib Dems, Lord Ashcroft's operation is widening its sights. Tory polling suggests the party is doing better in marginal seats than other constituencies. Its list of targets has been lengthened beyond the original 100 as the Tories, while publicly not taking victory for granted, try not just to win but to "win big".