"I may wake up tomorrow and do it. Then I may not," Richardson told The Associated Press.
The governor doubts the value of an endorsement in swaying voters although he acknowledged there's plenty of interest in political circles - including Clinton and Obama who frequently call him - in whether he throws his support behind one of them.
"The key is I don't think anybody cares," Richardson said of endorsements by politicians.
Richardson is the nation's only Hispanic governor. Hispanics are the nation's largest and fastest-growing minority and a key constituency for Democrats looking to win the White House.
Richardson had said on Wednesday that he would decide by the end of the week whether to make an endorsement. He said Sunday he remained torn.
"I'm usually not on the fence. But you know what, I like being on the fence. I am very comfortable with it," he said of his indecision.
There had been speculation that Richardson might make an endorsement before Tuesday's primaries in Texas and Ohio. Texas is heavily Hispanic.
If Tuesday's contests produce a clear leader in the race for convention delegates - Rhode Island and Vermont also vote - then Democrats should unite behind that candidate as their nominee to avoid an increasingly negative and divisive primary fight, Richardson said.
He did not define what he would consider a solid lead but said in the interview that it needed to be more than just a handful of delegates.
The governor first made that suggestion in a national television appearance Sunday.
"I just think the D-Day is Tuesday. We have to have a positive campaign after Tuesday. Whoever has the most delegates after Tuesday, a clear lead, should be in my judgment the nominee," Richardson said on "Face the Nation" on CBS.
