WASHINGTON — All along, the biggest question in this election was whether Republicans could take over the Senate and add it to their solid House majority.

Republicans need to gain six net seats to do so. Three, in states where Democrats are retiring, seem nearly certain: West Virginia, Montana and South Dakota.

Republicans’ chief targets elsewhere are Democrats running for re-election in states that President Barack Obama lost: Alaska, Louisiana, Arkansas and North Carolina.

Three other possibilities — Iowa’s open seat, and strong challenges to Democratic senators in Colorado and New Hampshire — give Republicans multiple paths to a Senate majority.

Five things to watch on election night:

CAN DEMOCRATS TOPPLE A GOP GIANT IN KENTUCKY?

Kentucky, often an early-reporting state, has seen one of the year’s most costly and fierce Senate races. From the start, Democrat Alison Lundergan Grimes faced a steep climb in trying to unseat five-term Sen. Mitch McConnell, who hopes to become majority leader in January.

Pundits like to cite McConnell’s relatively low popularity in Kentucky. But that ignores the extreme unlikeliness of Grimes getting votes from conservatives who find McConnell too accommodating and moderate.

Obama lost Kentucky by a whopping 23 percentage points. A Grimes victory would rank among the greatest upsets in modern politics.

WHAT ARE SIGNS OF A STRONG REPUBLICAN NIGHT?

If Republican nominees run ahead of expectations in Virginia and New Hampshire — even if they lose — it will be a bad sign for Democrats elsewhere.

Democratic Sen. Mark Warner of Virginia is expected to cruise past Republican Ed Gillespie. If Gillespie stays within, say, 5 or 6 percentage points, it could suggest tepid support for Democrats.

In New Hampshire, Democrats have always assumed Sen. Jeanne Shaheen would hold off Republican Scott Brown, a former senator from Massachusetts.

If Brown keeps the race razor-close, it’s a worrisome sign for Democrats. If Brown defeats Shaheen, Democrats can start packing their bags for the minority side of the aisle.

WHICH WAY WILL NORTH CAROLINA TIP?

North Carolina is one of the nation’s most closely divided states politically. Aside from Indiana, where Obama hardly tried in 2012, it’s the only state he won once and lost once.

Fittingly, the state’s Senate race has stayed relentlessly close.

First-term Democrat Kay Hagan proudly calls herself the Senate’s most middle-of-the-road member. She said her GOP challenger, Thom Tillis, speaker of the state House, led a conservative revolution that veered too far right for the centrist state.

This race may be too close for an early call Tuesday night. But both parties will watch intensely, not only for the Hagan-Tillis outcome, but also for hints of where a moderating southern state might trend in the next presidential race.

CAN DEMOCRATS GRAB GEORGIA’S GOP SEAT WITHOUT A RUNOFF?

Georgia Democrat Michelle Nunn and Republican David Perdue have made a few rookie mistakes in vying for the seat that Republican Saxby Chambliss is leaving, but they ran well-funded campaigns.

Obama lost Georgia by 7 percentage points, and it’s seen as a Republican state that might reach toss-up status in few years. A Nunn victory would vastly improve Democrats’ hopes of keeping the Senate majority. A Perdue win would let Republicans sigh in relief.

But the results might not be known for two months. With a third-party candidate running, a Jan. 6 runoff between Nunn and Perdue is possible. If that runoff decides which party controls the Senate, Georgia will see campaign spending like never before.

WILL SENATE CONTROL BE IN DOUBT FOR DAYS OR WEEKS?

Even without a Georgia runoff, it’s possible that Senate control will be in doubt when most Americans go to bed Tuesday night.

A Dec. 6 runoff in Louisiana seems likely.

The Alaska race between Democratic Sen. Mark Begich and Republican Dan Sullivan could be so close that the final tally could arrive with the East Coast’s sunrise Wednesday, or even later.

Late results from Louisiana and Alaska won’t determine the Senate’s control, however, if Republicans do well elsewhere.

Assuming they don’t lose in Georgia, Kentucky or Kansas, Republicans could secure the Senate majority with three victories among these competitive states: Arkansas, North Carolina, Iowa, Colorado and New Hampshire.