The old saying "put your money where your mouth is" requires people who support a certain course of action to do something active about it. Our good friend DovBear implicitly referred to this maxim when he counseled Mitt Romney's sons to join the war their father supports.
The obvious question is why Romney's sons should join the war. Who says they (or any politician's kin for that matter) support the Iraq War? Shouldn't we at least make an effort to determine their stand on that issue before asking them to join the military?
But let's use a hypothetical person, Person X, a young man who does support the war and see if the charge makes sense. Person X has always supported the Iraq War and believes we should keep the troops there until the job is done. Must he join the military and asked to shipped out to Iraq right away?
I don't see why. But first we must try to figure out what DB is actually claiming. He can't be making a descriptive claim because we all support claims and do nothing about them. I believe cancer research is quite important, but I've never donated a single dollar to a cancer charity. I may have good reasons for doing so (I might feel other charities are more valuable) or bad reasons (I might be too cheap to give charity) but either way no one can argue with a straight face that people generally support causes they feel are valuable.
So DB must be making a normative claim: if someone supports a cause he ought to do something active to help it. But why would that be? If I happen to believe a certain policy is the best morally or pragmatically, why must I do something to help that cause?
I'm not really sure, but I'd guess the answer has something to do with the implications of how the policy gets carried out. If Person X believes the Iraq war is important, he supports keeping our troops in Iraq, which requires us having troops in our military in the first place. Person X's preferred policy requires that some people join the military and go to Iraq. In other words, Person X's political view expects other people to carry out his policies. If that's true, perhaps Person X has the obligation see his views fulfilled, rather than expect other people to do things for him.
In all honestly, I don't find this line of reasoning very compelling (maybe because I made it up myself). But let's assume there is a moral obligation for a person who supports a cause to do something active to help it.
Let's use person X again, but this time let's assume he supports having a police force (this way we can avoid debating the Iraq War question). Person X always talks about how having cops walking the streets helps make society better. Does he have a moral obligation to join the police force? I'd say no for a few reasons.
First, while Person X has an objective obligation to help his personal causes, he might also have a subjective obligation to other people. Maybe Person X is married with two children and has an obligation to support his family. Becoming a police officer is risky and doesn't pay well, so Person X has to look for employment elsewhere if he expects his family to have a reasonable standard of living.
Second, perhaps his value to other causes is greater than what he can provide to the police force. Maybe he is a brilliant medical student who can help discover a cure for cancer, but is a physical weakling who would make a terrible police officer. So even if the police force is the most important cause, he can add more to the world by becoming a cancer researcher.
Third, while he values a police force, perhaps Person X considers other causes more important. Maybe he favors cancer research and has decided to devote his life to finding a cure. While a police force is important, his obligations to cancer research outweigh his obligations to the police force.
Lastly, maybe the marginal utility of his assistance to the police force is less than the marginal utility of his aid to another cause. Let's assume he considers the police force the most important cause in the world. The second most important cause is cancer research. But in this hypothetical world, there is a waiting list a mile long to join the police force, but no one wants to become a cancer researcher. So even if he feels he personally could help the police force more than the cancer research department at his local hospital, he might do the latter simply because his benefit to society will be slight if he takes the former route rather than the latter.
In summary I don't understand the source of this obligation, but even if it really exists, there are many reasons why someone might not "put his money where his mouth is."