Ministers want a consensus on party finance reform but this will be difficult
The Conservatives have criticised government proposals to reform party political funding laws as "half-baked".
The Tories said proposed changes to candidates' election spending were blatantly designed to help Labour MPs.
However, their amendment to deny the government's proposed bill a second reading was easily defeated.
Jack Straw told MPs that more openness about those giving money to parties was "essential" and urged cross-party consensus for reform of existing laws.
In the Commons, the justice secretary outlined plans requiring the source of any donation of more than £200 to be declared.
Search for consensus
But the Tories expressed dismay about plans for the existing £11,000 spending cap per candidate to apply to the entire period between elections not just the general election campaign itself.
This, they said, would give sitting MPs - who have an annual £40,000 "communications" allowance - a huge in-built advantage in marginal constituencies.
Shadow cabinet office minister Frances Maude accused ministers of trying to "force through major changes" to the law benefiting them at the next election without proper consultation.
Attempts to forge a cross-party consensus on donation and campaign spending levels broke down last year after Labour and the Tories failed to agree on recommended changes by Sir Hayden Philips.
The most controversial of these would have seen a cap on individual donations of £50,000 introduced.
In the Commons, Mr Straw said its proposed measures would bolster funding transparency and ensure "sensible controls on spending".
Under the proposals, anyone donating or lending more than £200 to a party will be required to declare the original source of the funds.
Scandals involving proxy donations to Labour, which are being investigated by the police, embarrassed the government last year.
Mr Straw urged the parties to find a consensus over the key issues of individual donations and spending limits, saying failure to do so would be a "further knock" to politicians' reputations.
"There is little purpose served in this house ramming through changes in party finances which don't command a consensus."
He added: "If we get into a party political dogfight on this, then any changes which will be made will be impermanent, they will not be properly enforced and the whole body politic will suffer."
But Mr Maude said the bill failed to deal with the "big donor culture" and described the proposed constraints on campaign spending as "outrageous" and "truly partisan".
"It cannot be right at the same time as we expand the scope for taxpayer-funded publicity for MPs - for the governing party to try to limit what candidates, who seek to unseat us, can spend from money they have raised privately."
The Lib Dems said the bill would not stop wealthy donors from "deluging" money into key constituencies and would not offer a "fair solution" to Labour's trade union funding link.
"We don't need tinkering, we need comprehensive reform," said its spokesman David Howarth.
The proposals would give the Electoral Commission tougher powers to investigate alleged abuses and the power to impose civil sanctions.
The Commission has welcomed these additional powers but warned proposals to allow political parties to nominate up to four members of the body would undermine public confidence in its independence.