WASHINGTON - Jeb Bush's announcement Tuesday that he is actively exploring a 2016 presidential run scrambles the large Republican field, thrusting him to the front of the pack and possibly locking up a huge swath of longtime party fund-raisers.

Bush, the 61-year-old son of one president and the brother of another, declared in a Facebook post Tuesday that he intends to set up a leadership PAC in January to "discuss the most critical challenges facing our exceptional nation."

His potential candidacy raises the prospect that the White House race will be a dynastic match between Bush and former Secretary of State Hillary Rodham Clinton, the heavy favorite for the Democratic nomination if she chooses to run.

But first, Bush would have to get through the primaries, where his conservative credentials are likely to be challenged by activists who scorn his support for immigration and education changes.

For now, the early move by the former Florida governor is expected to severely undercut the financial backing for other possible contenders - especially Gov. Christie, Texas Gov. Rick Perry, and Florida Sen. Marco Rubio. A Bush candidacy also could make it difficult for lesser-known potential candidates, such as Ohio Gov. John Kasich and Indiana Gov. Mike Pence, to gain traction.

Bush's early announcement could force others contemplating a bid - particularly Mitt Romney and Christie - to speed up their timetables.

"This frontloads everything," said Bobbie Kilberg, a top Republican fund-raiser in Virginia.

By midday, Kilberg said she had received calls from a dozen donors asking, "OK, what do we do now?"

Advisers to Christie did not respond to requests for comment.

One major advantage for Bush is the vast network of officials who served in past Bush administrations.

But other donors said potential Bush rivals were serious leaders who could attract their own financial support, a point made by Kenneth Langone, founder of the Home Depot stores and a Christie supporter.

"I'd let it play out a bit and learn a few things," Langone said, rejecting the idea that Bush's move put pressure on Christie.

Speaking to reporters at the Capitol, Rubio said: "I've got a lot of respect for Gov. Bush and I think he'd be a formidable candidate if he decides to run. But from my perspective, my decision is going to be based on where I can best advance my agenda for restoring the American dream."

A Perry representative sounded a similar message, and Wisconsin Gov. Scott Walker said Bush's moves would not affect his plans.

"The more the merrier," said Sen. Rand Paul (R., Ky.).

But there also were early signs of the backlash Bush would face among some on the right.

"I don't think we need another Bush. Period," Sen. Tom Coburn (R., Okla.), told reporters. "I like 'em all, but I don't think we need another Bush."

Democrats rushed to raise money off Bush's news, while top party officials scoffed publicly at the prospect of his candidacy. But privately, strategists close to Clinton have long viewed Bush as the biggest threat to her bid.