Sen. James M. Jeffords (R-Vt.) warned President Bush yesterday that he is close to leaving the Republican Party, and senior White House aides and congressional sources in both parties said they expected that Jeffords would do so and put control of the Senate in Democratic hands.

Jeffords, a moderate Republican who is often at odds with his party and who angered the White House by breaking with the president on his tax cut plan, met privately with Vice President Cheney and later with Bush at the White House. Jeffords said afterward that he plans to make an announcement today.

"I'm considering a lot of things," Jeffords told reporters at the Capitol. "Lots of people are trying to get me to do different things."

Both Democrats and Republicans said they expected Jeffords to quit the GOP -- most likely as an independent voting with Democrats -- to make Sen. Thomas A. Daschle (D-S.D.) the majority leader. A senior Democratic aide said last night that Democrats were "confident he will leave the Republican Party and join with Democrats in our caucus as an independent." But sources said Jeffords may not make the switch until after the Senate completes work on the tax cut and education bills pending before the chamber.

"All indications are he's going but may wait until after the tax bill," a White House official said last night.

Some cautioned that Jeffords could have a last-minute change of heart, and senior administration officials said they were holding out hope that he would do so.

A switch by Jeffords would have a profound impact on the balance of power in Washington, giving the Senate a 51-49 Democratic majority and posing new problems for Bush's legislative agenda.

As the new majority party, Democrats would control the flow of legislation in committee and on the Senate floor, regaining the committee chairmanships and leadership roles they lost when Republicans took over the Senate in 1994. The switch would have an impact on everything from the Bush administration's efforts to get its executive branch and judicial nominees confirmed to its success in pushing the president's legislative priorities.

It also would also end the landmark power-sharing agreement made by Daschle and Republican leader Trent Lott (Miss.) in January to reflect the 50-50 balance of power under which Lott controlled Senate floor action because of Cheney's tie-breaking vote.

Jeffords made no attempt in recent weeks to squelch speculation that he is considering a change in party affiliation. Democrats confirmed they have been making overtures to him, and a senior Democratic aide said last night that Jeffords would be given the chairmanship of the Environment and Public Works Committee. Senate Democratic Whip Harry M. Reid (D-Nev.), a key figure in the negotiations with Jeffords, is in line to become chairman of the committee but would step aside for Jeffords.

Confronting the threat of a Jeffords defection, some top Bush aides delayed their plans to attend a Republican National Committee gala to huddle in strategy sessions on how to keep Jeffords in the GOP.

Former senator Robert Stafford (R-Vt.), whom Jeffords succeeded in 1988, said Jeffords called him this week to seek advice. "I think he is very seriously considering what would be the wisest thing for him and for Vermont," Stafford told the Associated Press. "I have no idea from that conversation what he intends to do."

The Vermont Republican chairman, Patrick J. Garahan, said national party officials remained hopeful last night that "cooler heads will prevail" and Jeffords will decide to remain with the GOP.

Jeffords, 67, the son of a former chief justice of the Vermont Supreme Court, comes from an old New England family with deep Republican roots. But he has carved out an independent role in his 14 years in the House and 13 years in the Senate. He voted against President Ronald Reagan's tax cut in 1981 and supported President Bill Clinton's failed health care plan in 1993-94. But he has stuck with the GOP in power struggles with the Democrats.

Jeffords broke with Bush several weeks ago to join with moderates of both parties to force a reduction in the president's $1.6 trillion tax cut, scaling it back to $1.35 trillion.

After that, the White House passed over Jeffords when it issued invitations to a ceremony honoring a Vermont educator as teacher of the year. Senate sources have suggested that the administration might oppose a dairy compact that is especially important to Vermont.

However, the speculation over Jeffords's future has given him leverage to pursue his favorite causes, such as guaranteed funding for education of disabled children. The Senate recently approved the funding as part of Bush's education bill, which Jeffords is helping to steer through the Senate.

It also appears to have boosted his popularity in independent-minded and increasingly Democratic Vermont, where he won a third term by a 3-1 margin in November.

A month-old poll for the Rutland Herald showed that Jeffords's popularity was soaring and was considerably stronger than Bush's in the state.

Vermont Rep. Bernard Sanders, an independent, said voters would reward rather than punish Jeffords if he abandons the GOP. "The Republican Party today is much more conservative than Senator Jeffords or the Vermont Republican tradition," added Sanders, who said he has not spoken personally to Jeffords about the switch.

Earlier this year, Sen. Zell Miller (D-Ga.) started voting regularly with Republicans, spawning speculation that he would switch parties, which he quelled with a denial. Now, Republicans are mounting a strong new effort to get him to switch, according to a Democratic senator. Sen. Lincoln D. Chafee (R-R.I.), who is even more of a maverick than Jeffords, also was compelled to deny that he will switch. Sen. John McCain (R-Ariz.), who has his differences with the GOP and with Bush, is regularly asked if he will defect, and he keeps saying no.

A lifelong Republican in his third Senate term, Vermont's James M. Jeffords has often disagreed with the more conservative elements of his own party. He backs abortion rights and a variety of social programs. Story, Page A20.Sen. Thomas A. Daschle (D-S.D.) could become majority leader.