In the official report, NASA says that there was no pressure. However, not everyone agrees. Here’s a detailed article (possibly for a class originally, but with copious notes) which discusses the management process. THe launch had been postponed several times, and the engineers who built the boosters were overruled because they couldn’t prove that the launch would be unsafe, exactly the opposite of NASA’s previous SOP, that a no-go is issued if you can’t prove it is safe.
Also look at this page, specifically at the last few paragraphs. I haven’t been to the library to look up the Bell’s articles (they seem only to be online since 1989), but you can find the reference yourself and check it out. (Any of the Bell, 1987’s should link you to the reference page) I may do so also in the near future…
Regardless, the charge is that Reagan wanted to be able to talk about the Challenger in his State of the Union address, originally scheduled for January 28th, but postponed when the shuttle exploded that morning.