Jo Swinson has set out Lib Dem plans for a £5bn fund to improve flood defences across the UK, during a visit to flood-hit South Yorkshire.
She said the cash would help resilience to ensure the "heartbreaking" scenes of the past week would not be repeated.
Boris Johnson has chaired a government Cobra committee on Tuesday, to discuss the effects of the severe flooding in Yorkshire and the Midlands.
Labour leader Jeremy Corbyn said the Tories' response had been "woeful".
Mr Corbyn, who is also visiting the Doncaster area on Tuesday, said increased funding for flood defences and flood prevention schemes were "common sense" and would be priority for a Labour government.
"Just imagine if this had happened in Surrey rather than Yorkshire or the East Midlands," he said during an earlier visit to Blackpool, in which he repeated his call for the prime minister to declare a "national emergency". "I think it would have been a very different story."
Over the weekend, Mr Johnson visited Matlock in Derbyshire, one of the areas hit by severe flooding.
He said it was "almost certainly because of climate change," adding "we need to prepare and we need to be investing in those defences".
But opposition parties have said the PM has been slow to trigger a cross-government response through Cobra and release all the funding available through the Bellwin scheme, which provides emergency cash for local authorities.
"We now need a guarantee from the government… that money will be made available to all the local authorities that need it," Mr Corbyn said.
"We are in the midst of climate and environment emergency. Funding flood defences and emergency responders is the absolute priority."
During a visit to a community centre in Stainforth near Fishlake, Ms Swinson said the local response to the "devastating" floods had been heroic but the government needed to up its game.
"It is wrong that it seems just to be down to the local communities," she said. "This is clearly a national emergency and the government should be declaring it as such."
In addition to the £5bn fund, the Lib Dems are also pledging to increase the environment department's budget by £1bn per year.
Ms Swinson has faced questions over cuts to flood defence funding between 2010 and 2015, when her party was in coalition with the Conservatives and she was a minister in the business department.
And while volunteers involved in the rescue operation welcomed Ms Swinson's visit, one told the BBC he believed her presence was purely down to the election campaign.
"I can't ever remember a Lib Dem MP coming to Stainforth," Fred Turner, chairman of Stainforth4All, said. "No doubt it's political. No doubt about that."
But he added: "It's better than them not coming and seeing what's going on. They can see we're struggling and need help."
Chancellor Sajid Javid said the government had already activated the Bellwin scheme on Saturday but he agreed "much more" needed to be done to improve long-term resilience and water management.
"To do that, we have got to keep the public finances strong so we can invest them where needed, including on flood defences," he said on a visit to Bristol.
The Department for Environment, Food and Rural Affairs (Defra) is responsible for flood policy in the UK and gives funding to the Environment Agency to deal with problems with main rivers, estuaries, the sea or reservoirs.
Over the past 10 years, total spending on managing flood and coastal erosion risks in England increased by 18.4% to £808.2m in 2018-19.
Spending peaked in 2014-15 at £786.8m. Defra says a lot of that was down to repairing damage after heavy winter storms.