In the months following his re-election in November, President Obama seemed to be on a roll. During the "fiscal cliff" showdown that rang in 2013, he forced Republicans to do something they are especially loath to do -- accept a tax increase on higher-income Americans. With liberalism on the march, Republicans alternated between a pity-party and internecine warfare over who was to blame for their predicament.

Meanwhile, conservatives' marquee annual political gathering was evolving into a minor disaster, with no time to accommodate an accomplished conservative governor (Chris Christie) but a hearty welcome for an obnoxious, conspiracy-minded billionaire (Donald Trump), who will receive yet another platform from which to promote himself.

But as bad as things seemed, second-term presidents usually see the political tide shift against them quickly. It might have already begun last week. As The Washington Examiner's Conn Carroll notes today (page 26), the president's strategy of engaging Republicans on automatic spending cuts has begun to backfire. His dire claims about the devastating effects of sequestration are being disproved one by one, and citizens are noticing that the 2.3 percent cut in federal spending has failed to bring about the end of the world.

And the biggest shift so far came last Wednesday, when Sen. Rand Paul of Kentucky launched his epic 13-hour filibuster of John Brennan's nomination as CIA director. The episode instantly changed the political conversation in Washington and energized conservatives in a way unseen since last year's election. It was enough to shame the Obama administration, finally, into acknowledging that there are limits on whom presidents can assassinate and where.

It was also impossible to dismiss Paul's criticism as a mere right-wing attack, because a lot of liberals agreed with its substance. Paul also prompted many on the Right to rethink the Bush-era Republican attitudes toward executive power -- a change of heart that they should carry forward the next time a Republican occupies the White House.

The filibuster must have especially gotten under Obama's skin. He was elected on a promise to turn the page on the Bush era and conduct the war against terrorism with greater concern and sensitivity for civil liberties. But as president, Obama has shown that he thinks a muscular executive branch is OK so long as he's running it.

Paul has effectively reminded voters that the current president has not only failed to renounce controversial powers claimed by his predecessor, but has at times claimed new ones that go beyond them. Particularly illustrative is the White House's reluctance to renounce the authority to assassinate Americans within their own country absent any immediate threat. And Obama's ultimate admission that he lacks such authority vindicates the notion that one well-intentioned lawmaker, if sufficiently determined, can make a difference.