It would reshape the landscape and shake the Congress and White House - assuming it doesn't touch off a spate of GOP and Democratic moderates jumping ship for the other party. Reverberations and after-shocks could continue for years.
Suddenly, Democrats would control the Senate. Suddenly, the Bush administration's ambitious right-wing agenda would be in trouble. Suddenly, Hill Republicans would have to reconsider their hard-line approach.
GOP leaders knew this could happen in an evenly split Senate, but few expected a change so soon.
The focus - perhaps mistakenly - had been on 98-year-old Sen. Strom Thurmond and an ailing Sen. Jesse Helms. More attention should probably have been paid to Republican moderates such as Mr. Jeffords, who have received pretty shabby treatment from the White House since Mr. Bush charged into office.
The Bush administration has been demanding down-the-line loyalty from Republican majorities on Capitol Hill. When someone deviated from the party line, retribution was swift.
That proved a mistake in Mr. Jeffords' case.
For a quarter-century, the Vermonter has been known for his maverick streak and his liberal social stands. He sought concessions from the president on special-education aid in exchange for backing the Bush tax cuts. But the White House balked, and Mr. Jeffords opposed the larger tax cuts and helped craft the smaller proposal.
Congress isn't like Britain's Parliament. The White House never should have expected every party member to march in lock step with the president on every vote.
That was a costly miscalculation.
The president's advisers also decided to ram through as much conservative legislation as possible early in Mr. Bush's term before something happened to the GOP's two ailing senators.
That provoked an increasingly hostile response from Democrats and moderate Republicans. It's time for the president to re-evaluate his approach. Without a Senate majority, he'll have to.
During last year's presidential campaign, Mr. Bush repeatedly said he'd be a consensus-seeker with Congress. Now would be a great time to prove his sincerity - if he wants to get his legislation passed.
The entire Bush plan to name highly conservative judges, for example, must be overhauled: A Democratic-controlled Judiciary Committee won't stand for it. More middle-road Republican nominations will hold the best avenue for Senate approval.
A patients bill of rights, a higher minimum wage, a prescription-drug plan and the president's energy policy all could emerge from a Democratic Senate quite differently, thanks to Mr. Jeffords.
Not only the White House, but also gung-ho conservative ideologues in the Republican House may have to moderate their views and seek compromise solutions.
Bipartisan politics is back in vogue. Today's party-switching announcement from Mr. Jeffords could provoke a revolution.