CALVIN WOODWARD, Associated Press

Six is the magic number for Republicans bidding for Senate control in Tuesday's elections. That's how many they need to gain to become the majority.

About 10 Democratic seats are considered vulnerable to takeover by the GOP. A few Republican seats are also at risk.

In all, the magic six looked tantalizingly close. Republicans picked up three seats held by Democrats.

West Virginia came first as Rep. Shelley Moore Capito defeated Democrat Natalie Tennant in the race to succeed retiring Democratic Sen. Jay Rockefeller. In Arkansas, two-term Sen. Mark Pryor became the first Democratic incumbent to fall, defeated by freshman Rep. Tom Cotton. And in South Dakota, former Gov. Mike Rounds won a third seat for the GOP.

Montana, an open seat, appeared especially strong for Republicans. So, too, some of these seats held by Democrats: North Carolina, Colorado, Alaska and Louisiana.

Iowa, where Democrat Tom Harkin is retiring, also holds promise for the GOP.

But in New Hampshire incumbent Democratic Sen. Jeanne Shaheen fought off Republican Scott Brown to keep her seat.

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THE NUMBERS

On Tuesday, 36 seats are contested. Going into the election, Democrats held a 53-45 Senate majority, with two independents usually backing them. Tuesday's winners will serve for six years, until the end of the next president's first term.

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KENTUCKY HORSERACE

Senate Minority Leader Mitch McConnell, who stands to become majority leader if the GOP takes control, survived a spirited challenge from Democrat Alison Lundergan Grimes

Tons of money poured into the race between the veteran U.S. senator and Kentucky's secretary of state. Grimes faded in the homestretch.

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HISTORY IS MADE

In South Carolina, GOP Sen. Tim Scott easily won elections, becoming the first black senator elected from the South since Reconstruction

Appointed to the Senate after the resignation of Jim DeMint in 2012, Scott won the seat in his own right.

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KANSAS KINGMAKER?

In Kansas, independent Greg Orman is bidding to shake up the political calculus in his close race against Republican Sen. Pat Roberts. An Orman victory could even determine which party takes control, if the night produces a photo finish. Should Democrats come up with 49 seats and he aligns himself with them, they would maintain effective control. If he went with Republicans, he'd tip the balance to them.

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NORTH CAROLINA DONNYBROOK

First-term Democrat Kay Hagan, who calls herself the Senate's most moderate member, and GOP challenger Thom Tillis, speaker of the state House, put on a ferocious contest in one of the nation's most closely divided states politically. Aside from Indiana, where President Barack Obama invested little effort in 2012, it's the only state where he won once and lost once.

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KEYS TO CONTROL?

Their Arkansas hopes realized, Republicans also liked their chances against Sen. Mary Landrieu in Louisiana and Sen. Mark Begich of Alaska. If the GOP won those races, and secured South Dakota and Montana, along with their West Virginia victory, they'd have their six additional seats. That's if they don't lose some of their own.

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EYES ON IOWA

Republican state Sen. Joni Ernst faced Democratic U.S. Rep. Bruce Braley in a feisty contest enlivened with TV ads about castrating hogs. Senate Majority Leader Harry Reid of Nevada has said the outcome of that race is critical.

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DELAYED VERDICT POSSIBLE

It wasn't certain the key question of the election — who will control the Senate? — would be known by the end of the night, by the morning, for days, or conceivably for weeks.

Slow vote counts in Alaska could make Republican Dan Sullivan's challenge against Begich too close to call for some time. In Louisiana, many expected a Dec. 6 runoff between Landrieu and GOP Rep. Bill Cassidy. A Jan. 6 runoff in Georgia also was possible. A GOP wave would sweep away those uncertainties. Short of that, suspense might drag on.

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WHY IT MATTERS

A Republican takeover of the Senate, if combined with the expected continuation of a House GOP majority, would further complicate President Barack Obama's agenda in his final two years in office.

But it would also raise expectations that Republicans use their dual legislative majorities to govern, not just hold up what Obama wants to do.

Yet the dynamics that have produced so much gridlock in Washington would still exist. Obama could veto GOP legislation. Senate Democrats could employ the same delaying tactics on GOP initiatives that Republicans have been using against them.

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