This is true. That is how 500 votes in Florida in 2000 determined the Presidential election between Bush and Gore for Bush even when Gore won the majority of the popular vote.

In 1864 Lincoln won the popular vote by only 400,000 votes. Supposedly he won the "Army Vote" by 80%.

One of the legal questions in this vote was that there were 300,000 blacks in the Union army at the time. Where they allowed to vote? Legally they were not "U.S. citizens" and were now given the right to vote until the passage of the 13th Amendment in Dec 1865. Blacks were not the only "Non-citizen" element within the Union Army either. If these were allowed to vote, that would then create a multitude of additional legal questions.