Environment spokesman Don Foster said the party had made far-fetched assumptions about securing private capital.
Speaking about Labour's proposals for transport, which were set out by construction minister Nick Raynsford last week, Foster said: "The government has just made assumptions about the level of private sector spending. There is not evidence to suggest it is going to achieve it."
Foster pointed to Railtrack's poor stock market rating which he said would deter private capital.
In addition, Foster noted that the skills crisis was now so acute that it was a moot point whether Raynsford's construction pledge could be fulfilled. He said the decline in skills had got to the point at which it is unclear "whether we will be able to obtain the people we need to get this work done".
He also pointed to bureaucracy causing a log-jam in spending regeneration funding. He said: "There have been a plethora of different schemes introduced but they have their own bureaucracy and red tape. The complex bidding system has meant much of the money promised has been underspent."
Foster said the Liberal Democrats proposed a streamlined budget to span the different regeneration schemes, such as the Millennium Villages, the New Deal for Communities and the National Coalfields Programme.
He said the Liberal Democrats did back the government's attempt to outlaw cowboy builders and impose safety targets.
Foster's warning about spending was backed by Construction Products Association chief executive Michael Ankers. He said: "We remain sceptical whether the bureaucracy and planning will allow delivery on this scale, but we will certainly be pressing Labour very hard to deliver its plans if it is re-elected."
A Labour spokesperson said that if elected, the party was confident the £19bn would be spent over the next three years. She said: "We have created the economic stability to allow for more investment and intend to spend the money."
Foster spoke as the Liberal Democrats launched their transport strategy for the poll. The party proposes to restructure Railtrack into a not-for-profit body.