WASHINGTON — Senator Clinton’s official entry into the presidential race over the weekend, coming after months of coy hints and deflected questions, shook up the Democratic field, but none of her competitors appeared ready to concede the 2008 nomination to her.

“We’re a lifetime away,” a lesser-known Democratic rival, Senator Biden of Delaware, said yesterday on “Fox News Sunday.” “Hillary Clinton is going to have to make her best case. And there’s a lot of us out there that are known but in a sense not known, and we’re going to make our best case.

“And I don’t think Hillary’s best case versus mine or Barack’s or anybody else’s necessarily trumps us,” Mr. Biden said, referring to Senator Obama of Illinois, the popular Democratic upstart who formed a presidential exploratory committee last week.

Senator Kennedy yesterday stood by his commitment to his colleague in Massachusetts, Senator Kerry, saying on NBC’s “Meet the Press” that he would support the 2004 Democratic nominee if he decides to run again. Mr. Kerry has languished in the early polls, and Mr. Kennedy would not say whether he has encouraged Mr. Kerry to run, saying only that it was a “personal decision.”

Despite Mrs. Clinton’s candidacy dominating the airwaves, Governor Richardson of New Mexico went ahead yesterday with plans to announce his bid to become the nation’s first Hispanic president. Mr. Richardson, 59, was born in California to an American father and a Mexican mother, and he served under President Clinton as ambassador to the United Nations and later as secretary of energy.

Like Mayor Giuliani and Governor Romney of Massachusetts on the Republican side, Mr. Richardson is banking on a résumé filled with experience as an executive, rather than a legislator, as the clearest path to the White House. Although many have tried, no sitting member of Congress has won election to the White House since President Kennedy in 1960.

Previewing a campaign strategy on ABC’s “This Week” yesterday, Mr. Richardson offered a reason for that drought in his announcement, saying “most public policy solutions these days” are coming from governors and state government.

“On issues like the environment, jobs, education, and health care, governors are leading the way,” he said in a video statement on his Web site. “That’s because we can’t be partisan, or we won’t get the job done.”

In a crowded Democratic race in which more than a half dozen current or former senators may seek the nomination, only Mr. Richardson and Governor Vilsack of Iowa have been state executives. Mr. Richardson has the broadest résumé if not the longest, and he cited his experience as a congressman, U.N. ambassador, energy secretary, and governor to try to separate himself from his rivals. “Everybody talks about these issues; I’ve actually done it,” he said.

Mr. Richardson acknowledged that he does not have the celebrity status or the fund-raising ability of some of his opponents, but he said he could “outwork anybody.”

With the Republican field also taking shape, a former House speaker, Newt Gingrich, now says he will run only as a “last resort.” Mr. Gingrich, who led his party to control of Congress in 1994, had said he would consider entering the race by this September if no front-runner had emerged, but he qualified that statement in an appearance on Fox News yesterday.

The Republican front-runner, Senator McCain of Arizona, who supports President Bush’s plan to increase troops in Iraq, said yesterday that a nonbinding Senate resolution opposing the surge would amount to a “vote of no confidence” in America’s troops. Appearing on “Meet the Press,” he also said Mrs. Clinton would be a “serious president,” although he disagrees with her on many issues.