Election Day is here, and the main event is the battle for the Senate. Here's a quick way to understand the stakes and the trends that will develop over the course of the night - and maybe beyond - to determine which party will hold control.
ABC News is working off a list of 16 states (out of 36 total Senate seats on the ballot) that we see potentially swinging to a different party (starting with the earliest poll closings and finishing with the latest): Virginia, Georgia, Kentucky, North Carolina, West Virginia, New Hampshire, Arkansas, Colorado, Louisiana, Kansas, Michigan, Minnesota, South Dakota, Iowa, Montana and Alaska. Of the states on that list, 13 are held by Democrats now and only Georgia, Kentucky, and Kansas are currently held by Republicans.
Here are five things to keep in mind as you get ready to watch tonight's returns:
1. THE MATH
Republicans need a net gain of six seats to take over control of the Senate, which is now 55-45 in favor of the Democrats. Pollsters and party strategists agree that Republicans start the night at least halfway home because retiring Democratic senators in West Virginia, Montana, and South Dakota appear certain to be replaced by Republicans. Polling suggests that a fourth seat, in Arkansas, is tipping toward the GOP. That means Republicans could take over with just two additional victories out of a universe of 10 competitive seats now held by Democrats - assuming the GOP doesn't suffer an upset in a race or two where a Republican incumbent is playing defense.
2. THE BELLWETHERS
Two states with early poll closing times will provide an early glimpse into whether we're seeing a Republican wave. Polls close at 7:30 p.m. Eastern in North Carolina and 8 p.m. Eastern in New Hampshire. A loss by Democrats in either or both of those states would strongly suggest a wipeout of a night for Democrats, since incumbents have held narrow but consistent leads in both states. Also watch Virginia, with its 7 p.m. Eastern poll closing, for closer-than-expected results that would portend a huge night for Republicans. Democrats winning those three seats won't guarantee they can hang on to the majority. But losing any combination of them means the majority will almost certainly be lost.
3. THE FIREWALL
Democrats can actually lose the Senate if Republicans only win seats in states won by former GOP presidential nominee Mitt Romney two years ago. But they're almost guaranteed to lose control if Republicans pick up either or both of two significant bluish states, both of which carry special symbolic value for President Obama: Iowa and Colorado. Barack Obama carried both states in 2008 and 2012. But polls and early-voting figures point to strong GOP pickup chances in those states, suggesting a crumbling of the Democratic voting base this year.
4. THE WILDCARDS
Quirks and quirky personalities leave Democrats optimistic about their chances in two states few thought would be on the board this late in the cycle: Georgia and Kansas. The Democrat in Georgia has found late momentum in running against the business record of the Republican. Georgia requires a winning candidate to clear 50 percent, with a runoff triggered for Jan. 6 if nobody gets to that threshold. And in Kansas, the independent candidate is the main opposition to the Republican incumbent after Democrats succeeded in getting their candidate off the ballot. That independent hasn't said which party he'd join up with in Washington, but Republicans strongly suspect he'd be a Democrat in all but name.
5. THE LATE DECIDERS
Even without accounting for the possibility of multiple recounts, three states in particular could deliver overtime. Start with Alaska, where polls don't close until 1 a.m. Eastern and slow-voting and lagging absentee ballots could keep us without a winner for days or weeks. And Louisiana and Georgia both require winners to break a majority of total voters. Louisiana is almost certain to have a runoff, which would be held Dec. 6. Georgia's possible runoff would be a full month later. Republicans might need none of these seats to establish a majority. Or, they might need all of them.
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