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The fate of a clutch of endangered Republican incumbents, including some from states that heavily supported President George W. Bush two years ago, will determine who controls the Senate for the rest of the Bush presidency.
Both parties are spending millions of dollars in last-ditch efforts to influence those contests in Tuesday's elections.
By contrast, few Democratic incumbents are in danger this year. Nearly every Democratic senator seeking re-election seems assured of another term, polls suggest.
Not so with Republicans.
Facing difficult re-election battles are Republican Senators Conrad Burns of Montana, George Allen of Virginia, Mike DeWine of Ohio, Rick Santorum of Pennsylvania, Jim Talent of Missouri and Lincoln Chafee of Rhode Island.
Furthermore, Republicans face a strong challenge in Tennessee for the now-Republican seat held by retiring Senate Majority Leader Bill Frist.
Democrats also were hopeful for an upset victory over Arizona Republican Senator Jon Kyl, where polls have shown a tightening contest in recent days between the incumbent and former state Democratic Party chairman Jim Pederson, a shopping mall developer.
Democrats need a gain of six seats to win back the Senate, which they last controlled in 2002. Republicans now control 55 seats in the 100-member chamber. Thirty-three seats are on the ballot Tuesday.
Republicans are facing headwinds that include an increasingly unpopular war in Iraq, a president with low approval ratings, voter disgust over a slew of congressional scandals and the historic pattern of midterm losses in the sixth year for the party holding the White House.
"The political environment has been very tough for Republicans for many, many months," acknowledged Senator Elizabeth Dole of North Carolina, who heads the Senate Republican campaign effort. "No question about it."
Republicans picked up Senate seats in both the 2002 and 2004 elections. But strategists in both parties expect Democratic gains on Tuesday. Even if Democrats don't reclaim the Senate, they are clearly within striking distance of doing so.
And any pickup will increase their leverage in a chamber where strict rules dictate that 60 votes, rather than a simple 51-vote majority, are needed to advance any controversial piece of legislation or nomination.
With time running out, voter turnout has become the focus of both parties.
"The Bush team has been magnificent at turning out the vote in the last three elections. And this is their final challenge," said Republican strategist Scott Reed.
Senator Charles Schumer of New York, chairman of the Democratic Senatorial Campaign Committee, said, "The big battlegrounds are no longer Democratic states in the Northeast and the Middle West, but have become the border states of Missouri, Tennessee and Virginia.
"And I think that shows the trend being in our direction. A year ago, nobody would have thought that," Schumer said.
In the final days, "this is an election that is about the intensity of our base versus the intensity of their base," said Republican pollster and strategist Ed Goeas.
For Democrats, the Iraq war is providing most of the momentum. "It's generated their money, it's generated their activism." But on the Republican side, the threat of terrorism, taxes and fear of Democratic leadership of Congress are the big motivators, Goeas said.
Despite Democratic optimism over the possibility of a sweep on Tuesday, some party elders were cautious.
"There's a huge anti-Republican wave out there, but that wave's going to crash against a fairly stable, very stable, Republican political structure," said Democratic pollster and consultant Mark Mellman. "And so, it's a lot easier for Democrats to get votes than for Democrats to get seats."
"I have no doubt we're going to get a lot of votes, but getting seats is a harder nut to crack," Mellman said.
Nearly all the endangered Republicans would have to lose for Democrats to prevail. And Republicans were hopeful that Allen in Virginia and Republican Bob Corker in Tennessee would be able to win, citing a narrowing of the polls.
Democrats are fairly confident of picking up Republican seats in Ohio and Pennsylvania.
But their hopes of also picking up now-Republican seats in Missouri, Rhode Island and Montana were going down to the wire after a concerted Republican counteroffensive in those states.
Also, Democratic Senator Bob Menendez in New Jersey found himself as probably the only Democratic incumbent on Tuesday's ballot locked in a tight fight. He faced a strong challenge from Republican Tom Kean Jr., namesake of a popular former governor.