It was totally expected — her husband and daughter had taken the same position months earlier — and did not have as much political import as the announcement this month by Senator Rob Portman, Republican of Ohio, that he now favors same-sex marriage.
The rules are different for Mrs. Clinton. No nonincumbent in the history of contemporary U.S. presidential politics ever looked so formidable three years before an election.
Ask almost any Democrat and the automatic assumption is that Mrs. Clinton will be the party’s 2016 nominee; a top West Virginia Democrat says she would carry that state, which President Barack Obama lost, 62 percent to 35 percent, in 2012. Ask most Republicans who has the best shot to be the 45th president, and they will acknowledge it is Mrs. Clinton.
In conversations last week with more than half a dozen Clinton associates — people who know her very well politically or personally — there was a consensus: She is more likely to run than not. The presidential bug has not left her, and she passionately wants to see a female president. Still, her candidacy is not a foregone conclusion; those who say they know, do not; and there are several pressing questions outstanding.
First is her health. She just went through a fairly serious illness in December that sidelined her for a month. She developed a blood clot, an ailment she had suffered at least once before. Doctors who stress that they have no knowledge of her particular condition say a pattern of clots is worrisome.
She will turn 69 a week before the 2016 election — younger than Ronald Reagan was in 1980 or John McCain in 2008, and questions about her age reflect sexism. But questions about whether she can still bring her extraordinary vibrancy to any political task do not.
Then there is the health of the country in a few years. If the economy continues to improve and the world is relatively stable, her credentials to succeed and expand on Mr. Obama’s record, to build a more prosperous middle class and enhance America’s global standing, will be potent.
However, if the world economy deteriorates and U.S. unemployment hits double digits again, or if there is a conflict in the Middle East or on the Korean Peninsula, all bets are off.
Over the past 60 years, only once — in 1988 — has an incumbent party been given more than eight consecutive years in the White House.
Finally, what did she learn from the 2008 primary run? It was a disaster. She sought to inherit the nomination rather than capture it. She ran as a tough, hawkish, establishment candidate when most Democrats wanted an antiwar, anti-establishment nominee. Her campaign dissolved into chaos as she stubbornly refused to change. Ultimately, she dumped her campaign manager and the chief strategist of the flawed effort; but it was too late.
Would she pick better people next time and master the profound changes in political elections? She might start with Sasha Issenberg’s book “Victory Lab,” which describes the analytical revolution in U.S. politics. In 1991, there was a so-called Carville primary, as Democratic contenders vied for the services of the hottest political consultant, James Carville.
The equivalent this time might be a Teddy Goff primary, to earn the assistance of the 27-year-old digital wunderkind who directed social media for the Obama campaign.
In addition, unlike in 2008, Mrs. Clinton would have to adopt a strategy to fit the times. There is no need for her to rush.
She is selectively staying in touch with key players and is expected to write a book, settle into a role at a nonprofit organization focusing on global and domestic issues of prime concern, and get some rest.
One of the reasons Democrats are so eager for Mrs. Clinton to run is that they have a weak bench. Vice President Joseph R. Biden Jr., popular within the party and a loyal and important Obama lieutenant, will be almost 74 on Election Day 2016. That is four years older than Mr. Reagan was in 1980, when he was the oldest man ever elected president.
The animosity that Governor Andrew M. Cuomo of New York has incurred from more than a few national Democrats, and his controlling, secretive ways, would be a challenge in a national campaign. Governor Martin O’Malley of Maryland, attractive and articulate, does not seem ready for prime time.
Democrats have a long tradition of upending front-runners by picking nominees like George S. McGovern, Jimmy Carter and Mr. Obama. Hierarchical Republicans usually go with the establishment candidate. Thus, the cliché that Republicans like to fall in line and Democrats like to fall in love.
With Mrs. Clinton — who has rebounded from a defeat better than any recent presidential aspirant, including Mr. Reagan post-1976 — Democrats want to fall in line. Republicans, meanwhile, are desperate to find a candidate to love.
There are more than 1,000 days to go before any votes are cast. Another cliché is more telling: In politics, that is an eternity.