Now Gillibrand is popular with the party base, a reliable progressive, and one of the top names on the list of non-Clinton Democratic women whom people discuss as potential presidential material.

The Clinton decision may have been the glitziest one, but it's far from the only Obama cabinet appointment that had cascading, and sometimes controversial, political consequences. By asking then-Governor Janet Napolitano of Arizona to take over the Homeland Security Department in 2009, Obama handed the governorship to Republican Jan Brewer, then the state's No. 2. That gave the GOP unified control of Arizona's government; the party already had both chambers of the state Legislature, but most everything still had to get through Napolitano. One Democratic state senator criticized Napolitano for abandoning the state by taking the cabinet post.

The next year, Brewer signed the harshest immigration law in the country, Senate Bill 1070, which Obama called "irresponsible" and said "undermine[d] basic notions of fairness." That's just the best-known of many pieces of conservative legislation the state GOP pushed with Brewer in the governor's office. Napolitano's successor also slowed the advance of other ambitious state Republicans, who may have been eyeing the planned open governor's race in 2010.

By contrast, former Senator John Kerry's move to the State Department in 2013 opened doors for several Democrats in Massachusetts' static political hierarchy to move up.

Then there's Colorado, from which Obama poached former Senator Ken Salazar to be his first Interior secretary. The Democratic governor in turn appointed little-known Denver Public Schools Superintendent Michael Bennet to the Senate, kicking off a Democratic civil war in the 2010 primaries, a hard-fought general election that Obama's political advisers saw as a model for what they had to do in 2012. Now, as chairman of the Democratic Senatorial Campaign Committee, Bennet has become one of the Senate's more powerful Democrats. (His brother James Bennet is editor in chief of The Atlantic.)

The big political question about Obama's latest cabinet picks is how joining up will affect Castro's prospects and those of another bright former Democratic mayor, Transportation Secretary Anthony Foxx. Castro has long been seen as one of Texas Democrats' most promising rising stars, with a potential run for statewide office somewhere in the not-too-distant future. But even though the state's demographic trends look promising, Texas is inhospitable territory for Democrats right now. There is risk attached to an ambitious politician removing himself from politics, but Castro's near-term options in his state looked limited. Now that he's joined the cabinet, some say he could be a vice presidential pick for Democrats in 2016.

"I think there's nothing but upside here for Mayor Castro," said Matt Angle, a Texas Democratic strategist.