J.J., I understand that argument. Your original sentence still doesn't make sense though.
Well, Meeker and BRTD didn't so I had to explain it slowly.
If the LibDems come in second, even in terms of popular vote, the may (and I suspect will) be seen as the main opposition party, especially if the Conservatives form a government.
Elections can be about change in some cases, Home in 1964, Callahan in 1979, Major in 1997, and possibly Brown in 2010. In those elections, people looking for change looked at Lib/LibDem and, even if they liked the party/potential prime minister, also thought that the party didn't have a chance of effecting that change, by governing. That served as a reason
not to vote for them. Simply put, Lib/LibDem couldn't win.
Now, if it suddenly looks like it
could win, that argument goes away.
Further, some Labour voters, in particular, may decide, in the next election, that Labour cannot win, and vote for the party of change, LibDems.
It all depends on if the LibDems
look like they can win nationally. Driving Labour to third place in terms of the popular vote, would help with that perception, possibly enough to really cut into Labour seats.