Ezra Klein points to some recent research showing that there's been a trend over the past few decades for Congress to spend ever more time on presidential initiatives. It's up from about 15% of Senate votes in the early 80s to 25% today:
If you're wondering why this matters, the answer is simple: polarization. When the president takes a position on an issue, that issue polarizes instantly. To test this, Lee looked at "nonideological" issues — that is to say, issues where the two sides didn't have clear positions. In the Senate, only 39 percent of those issues ended in party-line votes. But if the president took a position on the issue, that jumped to 56 percent. In other words, if the president proposed the "More Puppies Act," the minority is likely to suddenly discover it holds fervently pro-cat beliefs.
So: more presidential initiatives, more polarization. Or is it the other way around? Has increased polarization forced presidents to be more proactive setting the legislative agenda — or, at the very least, forced presidents to take a public stand on more issues? Seems to me that could play a pretty big role in this dynamic.