Why is it so hard for President Obama to stick to a theme?

Wednesday afternoon provided a vivid juxtaposition that underscored this president's on-again, off-again relationship with message discipline.

On the House floor, 25 Democrats interrupted debate on a spending bill, coming forward one at a time to ask Republican leaders to take up an extension of unemployment benefits, which lapsed last month. The previous day, Senate Democrats had been doing their part to keep the issue prominent, provoking Republicans to block the legislation with a filibuster.

This is exactly the sort of time when presidential leadership is most effective, when consistent use of the president's megaphone can focus national outrage and force holdouts to relent. But at exactly the moment House Democrats were having their rebellion, Obama was giving a speech in Raleigh, N.C. — about wide band gap semiconductors.

"Wide band gap semiconductors, they're special because they lose up to 90 percent less power," he informed his audience as he announced that he was forming a high-tech manufacturing hub in Raleigh to produce such things. "They can operate at higher temperatures than normal semiconductors."

I'm all for wide band gap semiconductors, whatever they are. And the broader themes of Obama's speech — training, technology jobs and widening "the circle of opportunity for more Americans" — are worthy. In fairness, the president did call on Congress to extend the unemployment benefits — but he gave the topic all of 47 seconds, a third of the way into his speech.

We've seen this before on health care, gun control and other subjects: Obama will speak about a topic (as he did last week on unemployment benefits) and then move on before the job is done. But unemployment benefits should be a particularly easy sell for Obama: Republican opposition to helping job seekers (unless the money is taken from somewhere else) makes them sound heartless.

The House on Wednesday was debating the 2014 spending bill. It was being rushed to passage (as if to emphasize the haste, it was attached to a bill called the Space Launch Liability Indemnification Extension Act) and it was full of goodies for defense contractors and other powerful interests. But the alternative to passage was a government shutdown, and so the measure sailed through with only token opposition.

The absence of controversy on the spending bill gave House Democrats the opportunity to draw attention to unemployment benefits.

"I ask unanimous consent to bring up H.R. 3824 to end the Republicans' refusal to extend unemployment benefits that affect 355,000 in my [state]," said Rep. Eric Swalwell of California.

"I ask unanimous consent to bring up H.R. 3824 to end the Republicans' refusal to extend unemployment benefits that protect 49,965 workers in Michigan," said Rep. Daniel Kildee.

"I ask unanimous consent to bring up H.R. 3824 to end the Republican majority's refusal to extend unemployment benefits that would protect 137,315 workers in my home state of New York and that number is growing as we speak," said Rep. Paul Tonko.

Each time, Rep. Tom Cole, R-Okla., who was controlling the Republicans' floor time, was forced to raise an objection to the request. At the end, Cole thanked the Democrats for "the opportunity to renew so many acquaintances with my good friends on the other side and to make new ones." But Cole also felt the need to defend the Republican refusal. "Supposedly we're in the fifth year of a recovery and we have extended these extraordinary benefits for five years at the cost of hundreds of billions of dollars," he said, repeating GOP leaders' position that Democrats had to find a way to pay for another extension.

This was an argument Democrats wanted to have.

"Nobody talks about pay-fors for tax cuts for Donald Trump or subsidies to big oil or any special deals for corporate donors to the Republican National Committee," answered Rep. Jim McGovern, D-Mass. "But when it comes to extending benefits to unemployed Americans, we've got to find pay-fors?"

Rep. Sander Levin, D-Mich., called it "unconscionable" that Republicans are shutting down the House on Thursday for another recess while giving the jobless their "cold shoulder."

And Rep. David Cicilline, D-R.I., pointing out that 2.3 million children live with a long-term unemployed parent and that 1.5 million long-term unemployed workers have already been cut off, called it "outrageous that the House of Representatives would leave town again without taking time to renew this critical program to help struggling American families."

Outrageous. Unconscionable. Strong words, but valid — and missing from Obama's speech about semiconductors.