I found that Lincoln was a Democrat in my History book. Is it true?

nope he is a republican I don't know why he is a democrat

Explanation:

He was actually the first republican president :/

The party he participated to found was called "National Union party" in other words the republican party.

ummm? is that supposed to be a question or a but this is a reason he won re-election

explanation:

"despite progress in the war, lincoln and most political pundits were convinced that he would lose his bid for reelection in 1864. the country was war weary and the democratic party's nominee, george mcclellan, was likely to negotiate a peace treaty with the confederacy if elected." - loc.gov

google it and it will pop up