When the 2012 election cycle began, 23 of the 33 seats up for grabs were currently occupied by Democrats and Democrats were retiring in Nebraska, North Dakota, Hawaii and New Mexico, creating some tantalizing opportunities for the Republican Party.
After all, all the GOP needed was a net gain of four to guarantee a majority of 51. Right now, counting two independents who caucus with the Democrats, the Senate split is 53-47.
But despite all the computer modeling, the GOP wound up with some tighter-than-expected races.
In Missouri, U.S. Rep. Todd Akin looked to be a good match against vulnerable Democratic Sen. Claire McCaskill -- until he suggested there were "legitimate rapes," pushing public opinion in McCaskill's favor in recent weeks. However, after first abandoning him, Republican leaders and donors have begun signaling their support for Akin once the last week's deadline passed for him to drop out.
Democrats also have some candidates who have doggedly defied all the computer modeling that says they should have been in the weeds but remain competitive -- such as Joe Donnelly in Indiana, who faces Tea Party favorite Richard Mourdock to replace the party-ousted veteran lawmaker Richard Lugar.
All this means lots of tight races. The New York Times political blog Five Thirty Eight gives Democrats a 70 percent probability of keeping Harry Reid of Nevada as majority leader.
One of the most vigorous races is in Massachusetts, a decidedly blue state where the GOP presidential hopeful once governed and was considered unlikely to win -- spelling trouble for first-term Republican Sen. Scott Brown, who wrested the seat long held by Edward M. Kennedy from Democrats' hands while heralding the rise of the Tea Party movement in 2010.
Brown has fallen behind Democratic challenger Elizabeth Warren, who helped create President Obama's consumer financial protection board as part of the financial reform legislation signed into law in 2010.
RealClearPolitics.com's average of polls indicates Warren has a 1.3 percentage point advantage over Brown and rates the race as a tossup.
Akin's missteps may have benefited Linda McMahon, a former wrestling executive who's keeping it close with Democratic U.S. Rep. Chris Murphy, the San Francisco Chronicle reported. While first running from Akin, Republicans and conservative super PACs shifted funding into McMahon's coffers in a race that has become one of the most anticipated in the country.
RealClearPolitics.com's average gives Murphy a scant 0.5 percentage point lead and ranks the race as a tossup.
In battleground Virginia, two former governors square off to be Sen. Jim Webb's successor in what had been a close race until recently. GOP presidential candidate Mitt Romney's drop in popularity in Virginia seems to have temporarily dampened Republican George Allen's Senate prospects over Democrat Tim Kaine. Recently, five polls indicate Kaine led Allen.
Kaine held a 4.4 percentage point lead over Allen in the latest RealClearPolitics.com average of polls and the race is rated a tossup.
But some observers and analysts aren't ready to write off a Republican-led Senate, with one analyst saying a lot could change between now and Election Day.
"I'm not ready to call this done and over," Jennifer Duffy told National Public Radio. "We seem to be in some period of transition. Whether it's permanent or not, we'll know in a couple weeks."
Duffy, who analyzes and rates Senate races for Cook Political Report, rates as pure tossups Senate races in Connecticut, Hawaii, Indiana, Massachusetts, Maine, Montana, North Dakota, Nevada, Virginia and Wisconsin. Six of those seats now are held by Democrats and four by Republicans.
The Five Thirty Eight blog said forecast modeling isn't especially fancy, but more like an "overwhelming number of Senate polls recently have shown the Democratic candidates' standing improving."
August is when polling began trending toward Democrats in Senate races apparently because of two major political news items, the blog said. Romney selected Rep. Paul Ryan of Wisconsin as his running mate and Akin made his comment about "legitimate rape," possibly reinforcing the view that Republicans hold very conservative positions on social issues.
Another factor muddying the Senate waters is Romney's comment during a fundraising event in Florida that 47 percent of Americans pay no taxes, consider themselves victims and don't accept personal responsibility.
New York Sen. Chuck Schumer said Romney's "47 percent" comment pushed Democrats closer to keeping control of the Senate, U.S. News & World Report said recently.
"It shows the Republicans and Mitt Romney are characterized by narrowness," Schumer said. "Mitt Romney thinks if he and people like him do well, that is all you need to make the country do well. He has no understanding of how most people live and that is not because he is rich. Rockefeller did, Kennedy did, Roosevelt did. It is because he is so narrow in his perspective and beliefs."
Countering that sentiment somewhat is South Dakota Republican Sen. John Thune, chairman of the Senate Republican Conference, who told U.S. News capturing the Senate is a "50-50 proposition."
"I would advise our Senate candidates to do what they need to do out there to win," Thune says. "Obviously, stick up for the things they believe in and carry their own message. I don't think you rely on anyone else to do that."