I believe that liberals in this country do have a right to such a thing. However if they do such a thing it should be fair and legal and the other mainstream party here in the United States of America should have the same exact right as well if they want to.
Liberals should use tactical voting to keep conservatives out of government if they want to. It is not necessarily a bad thing. I think that if they want to avoid having a conservative person in office then they should definitely vote for whoever the other candidate is, even if they don't particularly care for that candidate.
Elections are about politicians convincing voters about their policies and how each party's programs will benefit the voters if voted into power.The only approved method to do this, is by going on the campaign trail to deliver your message. Voters are then left alone to decide who it is,they prefer.
Liberals have a tendency to come out in good numbers on Presidential elections. They however do not like to come out in those same numbers on off year elections. Is it because they know that their district has been gerrymandered? Even so, they should come out in extra force if they do not want conservatives in office.
As tactical voting can mean many things, such as limiting the voters in certain areas I believe this is not the right thing to do. But if tactical voting is by how they vote in congress, then yes tactical voting would be good and liberals should do this to help their party.