Many progressives are reluctant to present their cause as a matter of morals in addition to a matter of facts, learning and science. The willingness - if not preference - of conservative to argue a moral case has helped them to one degree or another.
It's a good thing when progressives want more than only gut feeling of right and wrong, justice and injustice to guide them. However, they would be mistaken if they believe that feelings play no part in their reactions to issues. Humans - and a number of animals - have a sense of right and wrong. Few people who aren't poor themselves get involved in fighting poverty because cold logic has convinced them that the poverty of the unfortunate has undesired consequences for themselves. Most of them find that poverty is simply "wrong", "unjust" and/or "inhumane" - even if it does not impact them in any way other than that sense of wrongness.
Consider the issue of gun legislation. It seems to me to be different than most progressive causes. It's not about government waging wars, levying taxes in an unfair manner or social programs. It's not about one group discriminating against another, employer-employee relations or such. It's about a small percentage of gun owners acting irresponsibly or criminally. But most progressives don't just hear that a politician proposes gun legislation and say to themselves, "That's a good idea", sign a petition - then go back to working on marriage equality. Rather, when they hear about Sandy Hook, they might drop their marriage equality work to advocate for gun legislation. Perhaps, what would be a more typical progressive movement would be to hear about a disturbed individual shooting people and begin a campaign for free, easily available psychotherapy and psychiatric care.
Not as many progressives are as fervent about pesticides - although they are also for killing animals and could also poison humans intentionally or accidentally. Progressives don't tend to gather on the issue of outlawing fireworks because people get injured every year and occasionally a fireworks business blows up. Progressives don't tend to want stronger narcotics laws because pushers seeking profits end up causing some addicts ro die each year, or because addicts are known for robbing people and doing other dangerous things which sometimes result in deaths.
The fact is, many progressives just feel guns are wrong. Some don't like the idea of hunting. Some are just less comfortable using force (even for good causes) than conservatives are. Others have different bad associations with guns.
My point is that in the final analysis, most progressives react strongly on gun legislation because of a feeling towards guns, not just towards the tragedies they can cause. There's nothing wrong with having feelings and a moral sense about issues - at least not as long as it's not inconsistent with the facts. Just know it's your feelings that motivate you on the issue.
It's not wrong for progressives. It's time to appreciate that other people's reaction to issues are influenced by feelings. If you want others to do more than say, "That's a nice idea" when progressives propose something, you need to make them feel a sense of right and wrong, just and unjust, not merely give them facts. If your audience is undecided whether you have a "nice idea", giving them a sense of right and wrong will give them extra impetus to look at the facts - and to care about what the facts say.
Humans are intelligent. We'd like to think That's all that's involved in decisions. Not only isn't it the only thing that influences our point of view, but intelligence isn't what makes the difference between accepting that something is true, and being active on the issue - rather than spending a pleasant day at the museum. It's the difference between an inspiring speaker and an informative lecture. It's the difference between a memorable, stirring article, and an article that contains useful statistics. It's the difference between a candidate who can motivate more people to knock on doors, and a candidate you vote for because he has more campaign planks that are closer to you than the other guy.
It matters. And whether or not progressives use it, conservatives will.