WASHINGTON — A usually elusive 60-40 Senate majority is the Democrats’ target in Tuesday’s election. They are certain to make gains even if they fall short of those magic numbers needed to break Republican chokeholds on their agenda.

Democratic Senate candidates will benefit from any coattail effect a big Obama victory might bring, as well as from the already weak economy and an unpopular war.

Math also favors Democratic Senate candidates this year. With 35 of the Senate’s 100 seats on the ballot Tuesday, Democrats have fewer at risk than do Republicans.

After picking up six seats in the 2006 midterm elections to wrest control from Republicans, Democrats have the barest of Senate majorities — 51 seats under their control, including two occupied by independents.

But they are overwhelmingly favored to pick up GOP-held seats in Virginia, New Mexico and Colorado, three states where Republican senators are retiring.

And many Republican incumbents running for re-election are in difficult races. Included in that group is Ted Stevens of Alaska, the longest-serving Republican senator in history who was convicted earlier this week on seven corruption counts. He’s high on the list of top Democratic pickup opportunities, even though Alaska has long been a solid Republican state. Stevens spurned pleas by John McCain and party leaders to step aside rather than stay in the race against Democratic challenger Mark Begich, the mayor of Anchorage.

Also on the list of endangered Republicans: Sens. Elizabeth Dole of North Carolina; John Sununu of New Hampshire; Norm Coleman of Minnesota; Gordon Smith of Oregon; Saxby Chambliss of Georgia; Roger Wicker of Mississippi and possibly even Senate Minority Leader Mitch McConnell of Kentucky.

No Democratic-held seats appear in jeopardy, and just one Democratic senator, Mary Landrieu of Louisiana, faces a serious challenge.

Republicans have spent heavily in Louisiana, and felt that the GOP’s candidate, state Treasurer John Kennedy, was within reach. But recent polls suggest otherwise.

National Democrats are taking full advantage of the Obama campaign’s organizing skills and registration drives, especially in battleground states.

“We’ve worked hand in glove with the Obama campaign,” said Sen. Chuck Schumer, D-N.Y., chairman of the National Democratic Senatorial Campaign Committee.

As to a filibuster-proof majority, “It’s hard to get to that 60 and I don’t want to oversell it,” Schumer said. “But I do think we’re going to pick up a large number of seats and that’s going to make Democrats very happy.”

In a mirror reflection of that optimism, Republicans are glum. Sen. John Ensign, R-Nev., chairman of the National Republican Senatorial Committee, says there’s no question that the top of the ticket is affecting Senate races, along with the financial crisis. “It’s a fairly toxic atmosphere out there” for Republicans, he suggests.

He acknowledged that Demo-crats could still exercise firm control by just coming close to the 60 votes: “I think if the Democrats get to 57 or 58 seats, on a lot of issues they will be able to override a Senate filibuster, because they seem to be able to pick off a few Republicans on a lot of the particular issues.”

The shifting political landscape is no more dramatic than in the South.

In North Carolina, Dole — who served in both the Nixon and Reagan administrations and is the wife of 1996 GOP presidential candidate Bob Dole — looked at first like a safe bet in her bid for a second term.

But revelations that she has spent little time in North Carolina in the past year have allowed Democratic newcomer Kay Hagan, a state senator with financial backing from national Democrats, to make the race one of the nation’s most competitive. A late-in-the-race Dole ad suggesting her rival has ties to an atheist group may have backfired. Hagan, who is a Presbyterian Sunday school teacher, took offense and threatened to go to court for a “cease-and-desist” order.

In next-door Virginia, another red state won by Bush in 2000 and 2004, Obama has made an aggressive effort and is leading in recent polls. Former Democratic Gov. Mark Warner, a Senate candidate, has a commanding lead in polls over another former Virginia governor, Republican Jim Gilmore.

In Georgia, incumbent Republican Chambliss looked like another sure winner. But he’s being forced into a tight race by Democrat Jim Martin, a former state representative.

What changed? A heavy Obama organizing campaign in the state, as in the rest of the South, has resulted in the registration of hundreds of thousands of new voters. In addition, Chambliss angered conservatives by voting for the $700 billion financial bailout bill.

While Chambliss appears slightly ahead, a spirited challenge from a third-party candidate could throw the election to a high-profile Dec. 2 runoff.

Other close races:

n Minnesota, where Democrat Al Franken, the former “Saturday Night Live” writer and actor, appears to have pulled ahead of Republican Sen. Norm Coleman in a closely fought contest.

n Mississippi, where Wicker, the appointed GOP incumbent, is holding a slim lead to fill the remaining four years of the retired Trent Lott’s term against former Democratic Gov. Ronnie Musgrove.

n Kentucky, where McConnell, the GOP Senate leader, is being heavily targeted by national Democrats. Although McConnell is favored to hold the seat, the Democratic pressure has kept him in his home state instead of helping other GOP candidates.

The prospects that Obama might win the White House and Democrats gain an effective filibuster-proof majority is not lost on the nation’s business community.

The U.S. Chamber of Commerce has poured $30 million into an issue-ads campaign arguing against what it considers would be an antibusiness Congress under a Senate firmly controlled by Democrats.

“I think that with a Democratic president, the focus on the Senate becomes even more pronounced, because the Senate will really be the arbiter of the final outcome of legislation,” said Bill Miller, the chamber’s political director.