One-third of Senate seats are up for election this year and Obama says many states with contested races tilt to the Republican Party.
"In this election cycle this is probably the worst possible group of states for Democrats since Dwight Eisenhower," Obama said Tuesday on WNPR, a Connecticut public radio station. "There are a lot of states that are being contested where they just tend to tilt Republican, and Democrats are competitive but they tend to tilt that way."
In the 1958 election, Eisenhower's second midterm, the Republican Party lost 13 Senate seats to the Democrats and Democrats gained two seats from the newly admitted state of Alaska. It was the biggest party shift in Senate history.
Obama's downbeat assessment came the same day that Vice President Joe Biden predicted Democrats would lose seats but retain control of the Senate.
Of 10 of the closest Senate contests this year, seven are in states that Obama lost in 2012. And Republicans are heavily favored to replace retiring Senate Democrats in Montana, South Dakota and West Virginia.
Republicans need to pick up six seats to gain a Senate majority for the first time since 2006.
The White House said Obama conducted 14 radio interviews over the past two days, virtually all of them with urban radio stations with largely African-American audiences. The chats were designed to get voters to cast their ballots and aired in some of the main battleground states, including Pennsylvania, Florida, Illinois and Michigan.
He gave interviews to four stations in North Carolina where Democratic Sen. Kay Hagan faces a difficult re-election. He also spoke to two Wisconsin stations where Democrats are seeking to defeat Republican Gov. Scott Walker and two in Baltimore, aiming for Maryland voters in the tightening governors' race.
Biden said in a separate radio interview that he foresees Democratic victories in closely contested races in Alaska, North Carolina, New Hampshire and Georgia and in a runoff election in Louisiana.
He did not mention Colorado and Iowa, two other neck-and-neck races.
Biden also suggested that Kansas independent Greg Orman could beat Republican Sen. Pat Roberts. Orman has not said whether he would join Republicans or Democrats in the Senate, only that he would caucus, or meet with, the party that has a clear majority.
Biden, predicting a Democratic majority, says Orman "will be with us."