Perhaps I am misunderstanding your theory. It appears your explanation for the steady increase in debt is that liberals want to increase spending and conservatives want to decrease spending. I do not see how that is an explanation for the increase unless you also are assuming that Democrats are more proficient at increasing spending than Republicans are at reducing it.
My theory explains why it is harder for politicians to reduce spending than it is for them to increase spending.
I don't deny that self-interest can play a role. I say is that it isn't the only thing. Party identification and ideology are other factors. The Republican Party has been very successful in promoting an ideology that denigrates the usefulness of many government activities. But then there is the obvious fact that if they abolished Social Security and Medicare (as many Republican politicians want to do, given that they opposed them from Day 1), voters would be hit hard; heads would roll, maybe even literally. So they must come up with schemes that undermine SS and Medicare in less obvious ways, such as privatization.
The Democrats are split. Progressives defend programs that help the poor and middle class. Centrists reflect more corporate concerns, such as deficit/debt. That's why we saw a centrist Democrat, Bill Clinton, balance the budget.
The Republicans have shown very different behavior, depending on who occupies the White House. When a Republican is President, Republicans say deficits don't matter, nothing wrong with stimulus, etc. When a Democrat is President, that's when they rant about deficits and the need to cut spending. This pattern won't necessarily continue forever. For example, with the election of more Tea Party people, who are more ideologically inspired and less politically astute, the Republicans may indeed become more consistent in their efforts to cut spending.
Also, you have to take into account that most voters do not devote a lot of time to following politics. They aren't reading papers put out by think tanks. They mostly go by party identification. They agree with one party's rhetoric more than the other, as a package deal, and then vote accordingly. Or sometimes a policy affects them directly, and they vote to throw the bums out (or support them, but anger is more motivating than satisfaction).