WASHINGTON (AP) -- A vice president-in-waiting who becomes unable or unwilling to take office can be replaced by the presidential candidate with the blessing of his party, as long as that happens before the Electoral College meets in December.

However, once the Electoral College votes on a new ticket, the Constitution requires Congress to confirm a new vice president if that office should become vacant. The 25th Amendment, ratified in 1967, has been invoked twice, when Vice President Spiro Agnew resigned in 1973 and again after President Nixon resigned in 1974, elevating Vice President Gerald Ford to the presidency.

These prospects were being raised Wednesday after Republican running mate Dick Cheney underwent minimally invasive surgery to open up a narrowed artery in his heart. The 59-year-old vice presidential candidate has had a history of heart problems, suffering three heart attacks and undergoing quadruple bypass surgery.

Still, Gov. George W. Bush reaffirmed his faith in Cheney's health and ability to serve, and campaign aides brushed off questions as to whether the Texas governor should consider substitute candidates.

But if such a replacement had to be made in the short-run, before the electors meet on Dec. 18, the choice would rest solely with Bush. The Texas governor would likely turn to the pool of candidates he accumulated before deciding on Cheney, experts said.

"That would be a purely political and quite informal process," said Mark Tushnet, professor of law at Georgetown University.

The Republican National Committee would gather to ratify his choice, and Bush would direct the electors when they meet to cast their votes for his designated vice president.

Even though the GOP presidential contender could avail himself of that option, some observers believe that with the uncertainty over the outcome of the presidential election, the Bush campaign would push that question off into the future.

"There is every reason in the world for Gov. Bush to leave speculation of a replacement to a much later date," said Jonathan Turley, a law professor and veteran constitutional litigator at George Washington University.

"Even if Mr. Cheney were to withdraw from his position, it would make great political sense to do so after the inauguration. The Bush campaign hardly needs one more element of complexity in an already Byzantine process," said Turley

Under that scenario, the 25th Amendment would come into play. The president would nominate a candidate who then must be confirmed in a majority vote of both the House and Senate.

Such a selection would not likely face serious challenges on Capitol Hill "unless there was some big ethical problem," Tushnet said. Typically, lawmakers give the president great latitude on such a selection.

The office of the vice president has been left open 18 times, because of deaths, resignations and successions to the presidency.

When Agnew resigned in 1973, Nixon nominated Ford to be his vice president. Less than a year later, Nixon resigned, Ford succeeded him to the presidency and designated Nelson Rockefeller as vice president.

There also have been instances requiring the replacement of a vice presidential contender before an election.

In 1912, Republican William Howard Taft chose James S. Sherman to be his running mate for a second term. But on Oct. 30, just days before the election, Sherman died. Taft named Nicholas Murray Butler, the president of Columbia University, as his replacement.

The Republican National Committee, however, did not have time to meet prior to the election, so the members gathered afterward and ratified Butler's selection, even though Taft had lost the election to Democrat Woodrow Wilson.