Like pardons and executive orders, vetoes are among the cherished privileges of the Oval Office. Ike liked them. So did presidents Truman and Cleveland - and both Roosevelts.

But apparently not George W. Bush. In fact, well into the fifth year of his presidency, he has yet to issue a single veto.

It's a streak unmatched in modern American history, one that throws into question traditional notions of checks and balances.

Although the streak could end next month - Mr. Bush is threatening a veto if Congress eases his restrictions on federal funding for stem-cell research - the Bush era thus far underscores a historically high-water mark of collegial cooperation between Congress and the White House, experts say.

"We're pretty close to a parliamentary government," says G. Calvin Mackenzie, professor of government at Colby College in Watervillle, Maine, referring to Congress's close alignment with the executive branch. "We don't have much recent history with that."

Other presidents have enjoyed majority support in Congress. But few, if any, have gotten the level of disciplined backing that Mr. Bush gets from House and Senate Republicans.

"There is unusual coherence between Republicans in Congress and the president," Professor Mackenzie adds. "So there's very little getting to his desk that hasn't been pre-approved by the Republican leadership."

On many major bills that Bush has signed - No Child Left Behind and tax relief, for example - the veto was never a consideration because the White House itself had proposed the legislation. Yet on dozens of other bills, the president has become a rubber stamp for a spendthrift Congress, betraying his campaign image as a fiscal conservative, critics say.

"The notion of limited government and frugal government has been shattered by this administration, which cares far less about limited government than it does in building conservative government - a government with huge payoffs to corporate America," says Allan Lichtman, a presidential historian at American University in Washington.

The last time a president's party dominated Capitol Hill was in 1993 and 1994, the first two years of President Clinton's term. That period was also marked by zero vetoes, but for a very different reason. Unruly House and Senate Democrats failed to toe the line on Clinton's big-ticket proposals, such as nationalized healthcare, leaving him with few major bills to sign. Lack of party discipline nearly scuttled the North American Free Trade Agreement and his budget. By the end of his second term, Mr. Clinton had issued 37 vetoes.

By contrast, when passage of the Central American Free Trade Agreement was in doubt last week, Bush personally made the trip up Pennsylvania Avenue to help bring reluctant Republicans into line. CAFTA's passage, however, was a result due as much to increased party polarization as Bush's arm-twisting, experts say.

The veto, of course, is far better at stopping legislation than at advancing it. But the threat of a veto can steer a bill closer to a president's goals. The transportation bill Bush signed last Wednesday is a case in point.

In 2004, he threatened to veto any highway bill that exceeded $256 billion. This year, he redrew the line at $284 billion. The version originally proposed in the House was well over $350 billion. But the continuing threat of veto eventually brought the final price tag to $286.5 billion, a figure Bush could tolerate.

"For fiscal conservatives, it's frustrating to watch," says David Keating, executive director at the Club for Growth, a Washington group that advocates fiscal responsibility and lower taxes. "He's beginning to lose all credibility with these veto threats."

The word "veto" does not actually appear in the text of the Constitution, but its function is implied in Article I. Significantly, the first presidents used the veto sparingly, reserving its use for legislation they deemed unconstitutional.

By the 20th century, vetoes were being issued more frequently, and being used more often as a political tactic than as a constitutional filter. President Franklin Roosevelt issued more than 600 vetoes - and that occurred even with huge Demo-cratic majorities.

Bush, however, hasn't even used the veto on legislation he deemed unconstitutional, such as the McCain-Feingold campaign finance reform he signed in 2002. That can be read as a sign of weakness, says Matthew Spalding, an expert on American political history at The Heritage Foundation, a conservative think tank in Washington. "Veto power has withered away from disuse."

Others take the opposite view. "Presidents who use the veto a lot are weak," says Bruce Altschuler, a professor of political science at Oswego State University of New York, noting Gerald Ford's time in office.

"More-successful presidents use it as a negotiation tool. When Bush has gone to Congress with [veto] threats, he has been effective," he notes.

Still, Bush may have to rely on the veto in years ahead because presidential power typically wanes in a second term. "A president's second term is like an hour glass with the sand running out," says Stephen Hess, professor of media and public affairs at George Washington University in Washington.

Already, Bush has struggled to marshal his party troops behind plans to partially privatize Social Security.

A test of GOP unity could come next month, when Congress will consider a move to relax Bush's limits on federal funding for stem-cell research. Senate majority leader Bill Frist - who is believed to be eyeing a presidential run in 2008 - announced a break with the president just before the August recess last week, a sign that fissures in the Republican bedrock are already appearing.

"The veto is always there; it's the paddle on the wall," says Jack Pitney, a professor of government at Claremont McKenna College in Claremont, Calif. "Everybody knows it's there. That gives the president a lot of power, no matter the alignment."