Nick Clegg will tell voters today that he will use a strong Liberal Democrat poll result to secure electoral reform as he issues his "personal guarantee" to deliver a fairer voting system.

The Liberal Democrat leader has shied away from suggesting that ending the first-past-the-post system is a pre-condition for working with another party in a hung parliament. But, in a pledge made in newspaper adverts and leaflets today, Mr Clegg says he will use the support he earns on polling day to "deliver fairness".

Senior figures in the party now see it as crucial that the party earn second place in overall votes if it is to secure a "moral mandate" for reforming the system. "If that happens, the system will have been shown to be tested to breaking point and reform would be inevitable," said one.

Leaflets handed out at train stations this morning read: "This is my personal guarantee that I will use all the support you give me on Thursday to deliver fairness in Britain. I will use your votes to reform Parliament, to deliver a fairer voting system, protect your freedoms and give you the right to sack corrupt MPs."

Mr Clegg's pledge also outlines the party's other main demands, including a radical reform of the tax system and the banking sector. "Whatever the outcome on Thursday, I believe we should be prepared to work together to fix the terrible state of our public finances and ensure economic stability," he says.

After visits to Eastbourne and Durham today, Mr Clegg will hold a homecoming rally in his Sheffield Hallam constituency in the evening. Campaigning at the Frontline Church in the Labour-held Liverpool Wavertree constituency, he said: "We had David Cameron measuring up the curtains for No 10 yesterday and Labour politicians today telling people how they should vote – Peter Hain and Ed Balls telling people what they should vote against, not what they should vote for.

"I want you to vote with your instincts, for the future you want."

The party was also having to fight off last-minute attempts to undermine its policies on defence yesterday. The former chief of defence staff, Lord Guthrie, the ex-MI6 chief, Sir Richard Dearlove, and the former Metropolitan Police counter-terrorism co-ordinator, Peter Clarke, had warned that Liberal Democrat policies on Trident and Afghanistan were "a gamble".

However, Mr Clegg dismissed the men as "a bunch of retired establishment figures" who had made big mistakes in the run-up to the Iraq war. "I am not going to apologise for calling, for example, for an inquiry into the allegations that the security services made us complicit in torture," he told GMTV. "That's a very serious allegation against our best British traditions. I want to promote those traditions."