While Hillary Clinton plans to announce Saturday her support for Barack Obama's candidacy, Democratic party leadership is growing increasingly frustrated with her timetable -- rancor that could hurt her chances of salvaging the party's No. 2.

Clinton raised the ire of several high-powered Democrats after she refused to concede and acknowledge Obama's historic achievement Tuesday, when it became clear he had earned enough delegates to become the first black presidential nominee of a major party.

House Majority Leader Steny Hoyer told FOX News that he "hopes" Clinton's scheduled address Saturday, which would end her bid to become the first woman president, would be a "very positive statement of support."

"It would be in her best interest," he said, adding it would bring the party together.

In an e-mail to supporters released late Wednesday, Clinton wrote, "On Saturday, I will extend my congratulations to Senator Obama and my support for his candidacy. This has been a long and hard-fought campaign. ... I have said throughout the campaign that I would strongly support Senator Obama if he were the Democratic Party's nominee, and I intend to deliver on that promise."

Party leaders hope she does.

Congressional lawmakers from Clinton's home state of New York -- who were some of her biggest backers -- endorsed Obama as a group Thursday, signaling it's time to close ranks behind the presumptive nominee.

Maryland Rep. Elijah Cummings, a Congressional Black Caucus member, said a lot of Democratic lawmakers "are very disappointed that she did not acknowledge he had enough delegates to be the nominee. And we're not even talking about congratulations."

"People just became concerned," said Cummings. "We in politics learn how to lose and learn how to win."

House Democrats have admonished Clinton to be more definitive about her plans and her deference to the presumptive Democratic nominee. To do otherwise could hurt her chances of obtaining a powerful slot in the coming administration, perhaps even the No. 2 post. And her supporters are vigorously lobbying for Obama to tap Clinton as his running mate.

But the Clinton camp tried to douse any perception that she is mounting a campaign to be vice president by issuing a statement on the speculation Thursday.

"While Senator Clinton has made clear throughout this process that she will do whatever she can to elect a Democrat to the White House, she is not seeking the vice presidency, and no one speaks for her but her," spokesman Howard Wolfson said in the statement. "The choice here is Senator Obama's and his alone."

Chris Kofinis, former spokesman for John Edwards' campaign, told FOX News that Clinton will have to focus in the coming days on how she would want to factor into an Obama administration. And he said she has a chance to set things right with Democrats on Saturday.

"They're wanting to do this next step very right. The speech on Tuesday got panned by a lot of folks for not going far enough to recognize the historic significance of Obama's nomination," he said. "She definitely wants to portray ... the unity message on Saturday."

Obama, speaking in Bristol, Va., Thursday, reached out to Clinton, congratulating her for what he called "her great achievement."

"I know I'm a better candidate because I ran against her. She's tough. She is just an outstanding candidate and a great public servant," he said, adding "I stand before you as the Democratic nominee for president of the United States of America. I'm very proud of that."

Chief among those who have prodded Clinton to formulate and announce a specific exit plan are Reps. Charles Rangel of New York, Barney Frank of Massachusetts, and Norm Dicks of Washington. All three were on a conference call Clinton conducted with about 20 House Democrats who had supported her campaign. The consensus among those Democrats was that Clinton had to decide what to do "sooner rather than later."

Rangel had voiced displeasure with Clinton's Tuesday night speech because it failed to acknowledge Obama's historic achievement.

On the call, Clinton declined to commit to a specific date to suspend her campaign or endorse Obama. Lawmakers agreed not to discuss the conference calls with reporters, but Rangel, Frank and Dicks made separate appeals to Clinton to come up with a plan to leave the race and to begin to set it in motion, sources told FOX News.

Her waiting "puts the congressional delegation in limbo because we leave Washington without having endorsed Obama ... we have to be able to say that he's won and we now have got to bring about unity. Ultimately Hillary Clinton agreed with that," Rangel said.

Rangel said the decision to hold the mass Obama endorsement by the New York congressional delegation Thursday was not to pressure Clinton to concede sooner. He said it was simply because they go home on Thursday night and will not be around Saturday when she makes her announcement.

On Thursday he downplayed the party divisions.

"We're Democrats. Dammit to hell we fight," he said. "When it's over, we come together and go out there to win."

But even if Clinton congratulates Obama on a mission accomplished, she is unlikely to disappear altogether. Still uncertain are the options remaining for Clinton to retain her delegates and promote her issues, including a signature call for universal health care.

She is closing down her payroll, however. Her campaign team's last payday is June 15. They will then be unemployed.

Many of Clinton's supporters could find employment, however, if she were to be included as the vice presidential candidate, Those dubbing the pairing as a "dream ticket" say it would bring Obama her enthusiastic legions and broaden his appeal to white and working-class voters.

Rangel said Obama and Clinton need each other -- it would help him pick up her supporters if she were on the Democratic ticket, and she "needs to maintain momentum" as a national and international leader.

Black Entertainment Television founder Bob Johnson told FOX News on Thursday that he is still plugging away for Clinton to be Obama's running mate. He has asked the Congressional Black Caucus to state publicly who they think Obama's best option is for a vice presidential running mate.

"Hillary has part of the Democratic puzzle, Obama has part of the Democratic puzzle. I think the demographic pieces that she brings to his demographic voters gives us a certainty of winning the presidency. And that's the most important thing, and that's why I think the two should come together," he said.

"This is Obama's decision, and if his political calculus says I can win without Hillary because I don't want her to help me govern, that's his decision, he's won that right as the nominee. But my position from looking at it and the politics of reality, I think why take the risk if you can win with Hillary?" Johnson added.

Clinton supporter Lanny Davis also just signed on to VoteBoth.com, a group advocating for the joint ticket, as a senior adviser.

Obama has indicated he intends to take his time making a decision. On Wednesday, he named a three-person vice presidential vetting team that included Caroline Kennedy, daughter of the late President Kennedy.

Asked late Wednesday what he thought of Clinton deciding to acknowledge his candidacy, Obama responded: "Truth is, I haven't had time to think about it. This weekend, I'm going home, talk it over with Michelle and we're going on a date."

FOX News' Major Garrett and Chad Pergram and The Associated Press contributed to this report.