Will Republican wave hit Senate on Election Day? History offers clues.

On Election Day, all eyes will be on the tossup Senate races to see whether Republicans retake control of that chamber. If history is any guide, most of the states will tilt in one direction – in other words, there will be a wave, and almost certainly, a Republican wave.

“It’s not going to be, ‘Oh, Kansas went this way and Georgia went that way, and Iowa went this way, and then Colorado went that way. They’re going to fall in the tradition,” says Amy Walter of the independent Cook Political Report.

Her template for tradition is the last eight election cycles, looking at the tossup races the weekend before the election. “When you average it … 80 percent of the closest races break one way or the other. It’s not 50-50,” she explained last week at an event hosted by the Bipartisan Policy Center, a Washington think tank.

Nothing's over till it's over, and Democrats have invested huge resources in getting out the vote. 

Still, Republicans need only a net gain of six seats in order to take the Senate, and the political terrain favors them: an unpopular president, Democratic incumbents running in red states that went for Mitt Romney over President Obama in 2012, and issues such as terrorism and Ebola breaking their way.

What political observers are asking now is how big a GOP wave might be. That’s an important question that determines not only whether Republicans take the Senate (they already control the House and are expected to pick up even more seats this midterm), but also how they interpret the outcome.

Is the wave a tsunami that flushes out Democrats even from states that would seem friendly to them? Then Republicans will feel emboldened, and perhaps avoid the adjustments that some leaders say must be made if the party is to appeal to more single women and minorities and youths to win the presidency in 2016. 

Just like waves at the beach, political waves break late – as they are roll into the election, says Whit Ayres, Republican pollster with North Star Opinion Research.

Mr. Ayres recalls 1994, when he polled for Republican Bill Frist, who was trying to unseat Sen. Jim Sasser (D) of Tennessee. A week before the election, Mr. Frist was up by four points. By that Thursday, he had widened the gap to seven points. By Saturday, he was up by nine. He actually beat the incumbent by 14 points.

“He gained 10 points in one week, but unless you were polling over the weekend, you’d miss that,” Ayres said at the Bipartisan Policy Center event. “We’re starting to see hints now of a building Republican wave.”

Polling in recent days shows Republicans increasing their leads in states such as Kentucky, Iowa, Georgia, Colorado. In Louisiana, Republican Bill Cassidy is gaining on Democratic incumbent Sen. Mary Landrieu in an three-way race. And two Sunday polls show a race originally expected to favor incumbent Sen. Jeanne Shaheen (D) of New Hampshire over Republican Scott Brown, now a tossup.

It won’t be much of a surprise if Republicans win against Democratic incumbents in red states. The sign of a strong wave is if they win in states such as New Hampshire, Iowa, and Colorado (which went for President Obama in 2012 and 2008) or North Carolina (where Obama lost in 2012, but narrowly won in 2008).

Those races will be telltale signs, even if others – Georgia and Louisiana – head to a runoff or, come Wednesday morning, are still too close to call.