Hard to start on cold days

I have been having problems starting my explorer when its cold outside (below 25). When I try to start it, it wants to turn over but wont. I can only seem to get it to start when i turn over the key and press the gas pedal to the floor, then it turns over and stays running. It did this last december a couple of times then it warmed up here where I live (iowa) know it got cold last night (20 degrees) and this morning It did it again. So what causes this

I have been having problems starting my explorer when its cold outside (below 25). When I try to start it, it wants to turn over but wont. I can only seem to get it to start when i turn over the key and press the gas pedal to the floor, then it turns over and stays running. It did this last december a couple of times then it warmed up here where I live (iowa) know it got cold last night (20 degrees) and this morning It did it again. So what causes this

What year and engine?

sorry,

It's a 97 explorer with 4.0L sohc