WASHINGTON - President Barack Obama would have a far tougher time advancing his agenda if the Republican candidate wins the special election in Massachusetts on Tuesday to replace the late Edward Kennedy, a Democratic icon, in the Senate.

On matters from healthcare to judicial nominees, Obama would no longer be able to prevail in the Senate with the support of only fellow Democrats and the two independents who typically vote with them. He would need at least one Republican vote -- and that could be difficult.

A Republican victory would rock Washington and send tremors across the American political landscape.

Following are some questions and answers about the election.

WHAT WOULD BE THE IMMEDIATE FALLOUT?

If Republican candidate Scott Brown wins, Democrats would lose their 60-vote Senate supermajority that now lets them clear Republican procedural hurdles and pass legislation without a single Republican vote.

Once stripped of such a majority, Obama and his fellow Democrats would be forced to try to find common ground with Republicans and get at least one of their votes. Republicans have opposed most of Obama's agenda, including his bid to revamp the $2.5 trillion U.S. healthcare system.

WOULD DEMOCRATS STILL CONTROL THE SENATE?

Yes. Democrats would continue to decide what comes up for Senate votes and would still chair Senate committees.

COULD DEMOCRATS PASS BILLS WITHOUT A 60TH SENATE VOTE?

Yes. Before getting their 60th vote, Obama won passage last year of major legislation to expand a federal children's health insurance program and a $787 billion economic stimulus. He did so with needed Republican support.

HOW LONG HAVE DEMOCRATS HAD A SUPERMAJORITY?

Democrats got their 60-vote supermajority last July when Minnesota Senator Al Franken was sworn in after a marathon court challenge of his 2008 win over Republican incumbent Norm Coleman.

That was the first time either party had a 60-vote supermajority since 1979, when Democrats had 61.

If Republicans win Kennedy's old Senate seat, Democrats would hold 59 votes, which would still give them one of the biggest Senate majorities in the last half century.

WHAT ARE THE POLITICAL RAMIFICATIONS?

If Brown wins, the election would be seen as a repudiation of healthcare reform legislation, which has been a major factor in the race. Brown opposes the legislation while Democrat Martha Coakley backs it.

A Republican victory would also underscore widespread voter unrest, fueled largely by an ailing U.S. economy with a double-digit unemployment rate. Democrats know their own jobs could be at stake in the November election unless they manage to create more jobs.

WHEN WOULD THE VICTOR BE SWORN IN?

Massachusetts election officials have said that it would take about two weeks to certify the winner, clearing the way for a Senate swearing in. But Republicans argue it shouldn't take that long for the winner to be seated.

On November 7, 1962, Edward Kennedy was sworn in as a member of the Senate -- a day after he won a special election to replace his brother, President John F. Kennedy, according to the Senate Historical Office. Kennedy's certificate of election was dated November 21, the historical office said.

WHAT ARE THE OPTIONS FOR DEMOCRATS ON THE HEALTHCARE BILL

IF THEY LOSE IN MASSACHUSETTS?

Democratic leaders are attempting to move the healthcare legislation through quickly. Sensing problems afoot, Obama stepped up the pace of negotiations over the past week. Interim Democratic Massachusetts Senator Paul Kirk has vowed to back the healthcare bill while he remains in the Senate.

Democrats could try to win at least one Republican Senate vote. Likely candidates would be moderates Olympia Snowe and Susan Collins, both of Maine. Support from either would likely entail various changes, though.

IN ADDITION TO HEALTHCARE, WHAT OTHER MAJOR EFFORTS IN

CONGRESS COULD BE AT RISK?

An already uphill battle to win passage of legislation to combat global warming would suddenly become much tougher. Same with legislation to tighten regulation of the U.S. financial industry.

(Additional reporting by Ros Krasny; Editing by Will Dunham)

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