If no pro-gun people at all vote against Obama, that's a terrible statement. It may not affect an election, but it would affect perception.
What would the perception be if the headline in November read: "Obama easily wins Oregon's electoral votes, despite getting absolutely no votes from gun owners"? The perception would be that gun owners have less political clout than politicians feared, and maybe politicians shouldn't be so afraid of the NRA and gun owners. I don't think that's a perception that we want to encourage.
Taking it a step further, if Romney loses, and his loss is blamed in part because he got only lukewarm support from gun owners, maybe the Republicans will learn that they shouldn't nominate a candidate who is so questionable, flip-floppy, soft, and squishy when it comes to gun rights if they want to win next time.