Republican candidate for U.S. Senate Linda McMahon, left, shakes hands with Democratic candidate for U.S. Senate Chris Murphy at the end of a debate in Rocky Hill, Conn., on Oct. 7, 2012. (Jessica Hill/Associated Press)

In the past six years, there have been three national elections, each of them producing a wave, in which races up and down the ballot moved in generally the same direction. Democrats won big in 2006 and 2008, only to see those gains reversed in the mid-term election of 2010.

This year, however, the better analogy in many states may be a breeze. The presidential contest will ripple the waters but not necessarily determine which way they flow.

That’s partly because the presidential race is so close, and its battlefield is so narrow. Just four weeks before Election Day, fewer than 10 states are in play for the presidential contest, which allows for separate dynamics to take hold in House, Senate and gubernatorial races elsewhere.

There is also a paradox at work. While the amount of territory being contested in the presidential race is relatively small, “this is the largest Senate map that I can remember, certainly in a decade,” said Guy Cecil, executive director of the Democratic Senatorial Campaign Committee.

The same is true among House races, said Mike Podhorzer, political director of the AFL-CIO, though he said that could be because so many of them have been under the radar in this presidential election year.

Explore the 2012 Senate map and view unemployment and demographics

“It looks like suddenly a lot more races are in play,” he said. “But I think a lot of them have been in play all along, just people didn’t know about it.”

Many of the most competitive House races are taking place in what Speaker John A. Boehner (R-Ohio) calls “orphan districts,” places that the presidential campaigns are all but ignoring, and where there is not even a hotly contested Senate race.

And even in some of the battleground states, candidates say they are all but ignoring what is happening at the top of the ticket.

Former House member Charlie Wilson, one of a handful of ex-lawmakers seeking to return to Congress, said he cannot count on President Obama’s performance in Ohio to carry him over the top in his conservative southeastern district.

“I run my race, and he runs his, and I can’t mix the two,” Wilson said.

Once-a-decade redistricting also has changed the equation. In some of the battleground states, such as Ohio and Virginia, it has strengthened the hold that Republican incumbents have on their districts and made them less vulnerable to the outcome at the top of the ticket.

But in California, where only one House seat changed parties over the past decade, the newly drawn lines have created something the state usually doesn’t see in congressional elections: suspense.

This year, the district lines have been so altered by a new nonpartisan reapportionment process that at least four of California’s 53 congressional races — all of them in seats currently held by Republicans — are rated as tossups by the authoritative Cook Political Report.

In recent weeks, the political tide has turned ever so slightly in House Democrats’ favor, particularly in late September as GOP presidential nominee Mitt Romney stumbled.

Still, to gain the 25 seats they need to return to the majority in the House, Democrats probably need to defeat at least 35 Republican incumbents. That appears unlikely, although Republicans concede privately that their ranks are likely to be reduced somewhat.

“There’s going to be a lot of money moving around in coming weeks. We’re going to have to spend a lot,” said Paul Lindsay, communications director for the National Republican Congressional Committee. “At the end of the day, [Democrats] are not going to be anywhere near the number of seats they need.”

Meanwhile, in some Senate contests, candidates are actually defying the gravitational pull from the top of the ticket.

“They are affected by, but not determined by, the presidential race,” Cecil said.

Romney is expected to trounce Obama in North Dakota, for instance. But in that state’s Senate contest, former attorney general Heidi Heitkamp, the Democratic nominee, is running a close race against Rep. Rick Berg (R).

The opposite dynamic is taking place in Connecticut, a deeply Democratic state that Obama will win in a landslide.

The Republican Senate candidate, former World Wrestling Entertainment CEO Linda McMahon, lost her first race in 2010, which was a good year for Republicans. But McMahon put the lessons she learned from that defeat to good use — along with more than $13 million of her personal fortune. This year, she is in a surprisingly tight battle against Rep. Chris Murphy, the Democratic nominee.

But where Republicans were thought at one point to stand a good chance of regaining the Senate majority they lost in 2006, most handicappers now expect them to fall short.

In the governors’ races, Democrats are playing defense nationally. Eight of the governorships they hold are on the ballot this year, compared with only three for the Republicans. But only two of those races — in New Hampshire and North Carolina — are in states that are also competitive in the presidential contest.

Democrats also say they are confident that some of their candidates — particularly incumbents Earl Ray Tomblin in West Virginia and Jay Nixon in Missouri — are strong enough to withstand the Republican tide at the top of the ticket in those states.

Still another factor is at play in many states: 2012 is an unusually busy year for ballot questions, which can create a dynamic of their own.

Voters in Maine, Maryland and Washington state will decide whether to approve same-sex marriage. In Minnesota, by contrast, a ballot question would allow voters to explicitly bar same-sex marriage by declaring that marriage consists only of a union between one man and one woman.

Maryland also has two other landmark measures on the ballot. One would expand gambling in the state, allowing “table games” such as blackjack and roulette at the state’s existing casinos (now limited to electronic games), and permitting an additional casino in Prince George’s County.

The other measure would allow voters to decide the fate of legislation that would give in-state tuition discounts to young immigrants in the country illegally. The immigrants would have to prove they had attended a Maryland high school, that their parents have filed tax returns and that they intend to become citizens. They would also have to start their higher education at a community college.

In Colorado and Washington, voters could choose to legalize the possession of a small amount of marijuana. Oregon voters could approve a more expansive law, which would allow more pot to be sold, but only through state-run stores.

Also, a ballot measure in Arkansas could make that state the first in the South to legalize medical marijuana use.

David Fahrenthold and Ed O’Keefe contributed to this report.

Read more on the final four weeks of the campaign:

11 states face governor’s races

What will be this year’s ‘October surprise?’

Top 10 Senate races to watch

2012 House races to watch

Basic tips on watching the flood of polls