WASHINGTON (CNN) -- It may not grab the headlines like the race for the White House, but the battle for the Senate is heating up.
The retirements of Sens. John Warner of Virginia, left, and Chuck Hagel of Nebraska open up two key races.
Democrats took back both houses of Congress last November, but they hold razor-thin majorities. In the Senate, they hold a 51-49 margin. Their advantage in the House is a couple of percentage points better, 233-202.
With such a small edge, you would think Republicans would have a good shot at taking back the Senate in 2008. But the numbers tell a different story.
Of the 34 Senate seats being contested next year, Republicans hold 22. And with three Republican senators retiring and with four facing difficult re-election bids, Republicans have a tough task ahead of them in trying to recapture the upper chamber of Congress.
On Thursday, Republicans got some good news when Agriculture Secretary Mike Johanns announced he was resigning. A GOP source said Johanns has told friends he will run for retiring Sen. Chuck Hagel's Senate seat in Nebraska.
Johanns was in his second term as governor of Nebraska before stepping down in January 2005 to join President Bush's cabinet.
Hagel told CNN Wednesday that he had spoken with Johanns and encouraged him to run. Hagel also told CNN that Johanns would be making an announcement soon.
If he runs, Johanns may not have the Republican field to himself. Hagel's public criticism of the Bush administration's handling of the Iraq war had already prompted Nebraska Attorney General John Bruning to mount a primary challenge to the incumbent senator. Now that Hagel's not running, there are no guarantees that Burning will step aside for Johanns.
Two other Republicans are also in the hunt for the open seat.
Among Democrats, former Sen. Bob Kerrey is considering running for the open seat.
Nebraska's generally considered to be a red state. Bush handily won re-election there in 2004. But an unpopular war and an unpopular president could give the Democrats hope next year, even in the Cornhusker State. The state's other incumbent senator, Ben Nelson, is a Democrat.
A Johanns-Kerrey matchup would instantly become high-profile. But it's not the only one.
It's a similar story in Virginia, where longtime Republican Sen. John Warner is retiring next year.
Former Gov. Mark Warner, no relation, said that he intends to run for the open seat. Warner's term as governor ended in January 2006, and he left office with high favorable ratings. Warner then flirted with a run for the White House before deciding against it.
Virginia was once a reliable red state, but Democrats have won three major statewide elections there this decade.
In 2006, Democrat Jim Webb ousted Republican incumbent Sen. George Allen by a margin of fewer than 10,000 votes, or less than half a percentage point. Webb's victory helped give Democrats their slim Senate majority.
The race there next year could be equally close.
"Virginia has been a reliable Republican state since the 1960s with an occasional Democratic breakthrough," CNN senior political analyst Bill Schneider said. "Now those breakthroughs are becoming more frequent."
On the Republican side, there could be a bitter primary fight between Rep. Tom Davis, a moderate from northern Virginia, and conservative former Gov. Jim Gilmore, who dropped out of the presidential race earlier this year.
The other state where Republicans will be defending an open seat is Colorado. Sen. Wayne Allard announced earlier this year that he was retiring after his term finished and Democrats made major gains there in the 2004 and 2006.
In addition to those races, some Republican senators up for re-election next year will be fighting for their political lives -- Susan Collins of Maine, John Sununu of New Hampshire, Norm Coleman of Minnesota, and Gordon Smith of Oregon.
As of now, only one Democrat -- Mary Landrieu of Louisiana -- appears to face a major fight.
The New Hampshire race will most likely be in the national spotlight, now that former Granite State Gov. Jeanne Shaheen will take on Sununu, a first-term senator. The race will be a rematch of the 2002 election, in which Shaheen lost to Sununu in a very close contest.
And the Minnesota race could also be bitter. Coleman is also a first-term senator who squeaked into office in the 2002 midterm election. The Minnesota contest could also become even more interesting if comedian and liberal talk show host Al Franken wins the Democratic nomination.
It's not only the numbers that are stacking up against Republicans.
"You've got an unpopular war, an unpopular president and an overwhelming desire for change. And in a presidential year, the incumbent president's party defines the status quo," Schneider said.
Also hurting Republican candidates are the scandals involving and investigations into into several of their colleagues. Democrats coined the "culture of corruption" slogan last year, and it was one of the minor factors that helped them wrestle control of the House and Senate from Republicans. They are hoping that strategy works again in 2008.
The recent sex scandal involving Sen. Larry Craig should not hurt the GOP's chances of holding on to that seat in 2008 if the Idaho Republican does resign from office at the end of the month as he has said he will. At the least, though, the controversy over his arrest in a Minneapolis airport restroom and subsequent guilty plea won't help his party.
In most national polls, only around a quarter of Americans think the Democrat-controlled Congress is doing a good job. While that would appear bad news for Democrats, half of those responding to a CNN/Opinion Research Corp. poll said the policies of Democrats in Congress would move the country in the right direction while only 34 percent said Bush's policies would.
The numbers, for now, seem to be working in the Democrats' favor. But November 2008 is a long way off. E-mail to a friend