Actually he wasn't. The Democrats of the day were the small Govt. anti everything party. Lincoln was for the Feds doing everything from building a transcontinetal railway to paying for local infastructure improvements. Homestead Act, an a whole bunch of other expansions of the size and scope of the Fed Govt. To put him in terms of todays political debate, he would be a Moderate to Conservative Democrat.
Lincoln, in his time, was for more government intervention than the Democrats were. However, I think it would be unfair to call him a moderate or conservative Democrat in today's terms. The Democrats might have been more anti-government, but that doesn't make Lincoln a Democrat in today's terms. He knew the wisdom of limited government infringement upon the liberties of man.