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Only in Washington: Republicans need 50 seats to retain control of the Senate in Tuesday's elections, but Democrats must have 51 to take it away.

The Constitution (search), the calendar, the presidential election and a political power play in Massachusetts combine to make it so, a curiosity that gives Republicans a margin for election error that Democrats lack.

"If President Bush and Vice President Cheney are re-elected, if the Senate is 50-50, the Republicans would maintain control" by virtue of (Vice President Dick) Cheney's ability to break a tie," said Betty Koed (search), assistant Senate historian.

Even if Kerry wins the White House, she said in a recent interview, "The Republican vice president would be breaking the tie" from Jan. 3, when the Senate convenes, until 17 days later, when the White House shifted hands with the swearing-in of the next president.

By then, Koed said, Kerry would have to resign his Senate seat to take the oath of office as president. Once again, Democrats would be shut out of the majority in the Senate, left with 49 seats to 50 for the Republicans.

Barring the unexpected — Democrats finally prevailing on GOP Sen. Lincoln Chafee of Rhode Island to switch parties, for example — it would stay that way until spring or summer. That is when Massachusetts voters would elect a replacement for Kerry under a law approved by the Democratic-dominated Legislature. State lawmakers acted to block GOP Gov. Mitt Romney from naming a Republican to the seat.

The current Senate has 51 Republicans and 48 Democrats. The 100th senator, Jim Jeffords, is an independent from Vermont who sides with the Democrats for organizational purposes.

In all, there are 34 Senate races on the ballot on Tuesday, only nine of them viewed as competitive by strategists in the two parties. Democrats must win six to move into a 50-50 tie, and seven to win an outright majority.

Most incumbents in both parties are coasting to new terms after campaigns in which they drew little-known and poorly funded rivals.

Idaho Sen. Mike Crapo, a Republican, arguably has the easiest time. His only opponent is a write-in, Democrat Scott McClure.

Sen. Judd Gregg, R-N.H., is nearly as blessed, politically speaking. His opponent is Doris (Granny D) Haddock, 94, who once drew nationwide fame for her crosscountry walk in support of legislation to reduce the influence of big money in political campaigns. Her most recent report on campaign funding showed less than $10,000 in the bank.

Sen. Harry Reid, second-ranking Democrat in the Senate, is coasting, as well, six years after surviving a re-election scare by only 428 votes. Republicans tried to recruit a better known challenger, but failed — as they did in North Dakota, where Democratic Sen. Byron Dorgan is expected to win easily.

By general agreement of strategists in the two parties, two seats appear virtually certain to change hands.

In Georgia, Rep. Johnny Isakson is running to succeed Zell Miller, a Democrat. In Illinois, Barack Obama is favored by far to replace GOP Sen. Peter Fitzgerald and become the only black member of the Senate.

Obama, coasting in his own race, headed for Kentucky on Sunday to try to help Dan Mongiardo in his bid to upset GOP Sen. Jim Bunning. That race emerged in the final two weeks of the campaign as a competitive one — and not for the faint of heart.

Democrats have repeatedly called Bunning's mental fitness to serve into question. As his lead dwindled in the polls, Republicans responded by speculating openly about Mongiardo's sexual orientation.

"I think we've all broken the Ten Commandments," said Bunning on the campaign's final weekend, deflecting charges from Mongiardo that he had violated two of them. "No one is an exception to that."