One of the highest immediate political priorities for Senate Majority Leader Harry Reid (D-Nev.) and Senate Democratic political guru Charles Schumer (N.Y.) is to persuade veteran colleagues not to retire in 2014.

Democratic sources identify four senators as most likely to retire: Sens. Jay Rockefeller (D-W.Va.), Tim Johnson (D-S.D.), Frank Lautenberg (D-N.J.) and Tom Harkin (D-Iowa).

Another possible veteran retirement is Senate Armed Services Committee Chairman Carl Levin (D-Mich.), who has yet to announce his decision. But Democratic aides expect him to run again.

Democrats would have a tough time defending their seats in West Virginia and South Dakota without incumbents running. Iowa, a blue-leaning state that gave President Obama a six-point victory over Mitt Romney, is a competitive race that slightly favors Democrats and New Jersey is a safe bet to stay in their column. Michigan resembles Iowa in that it leans Democratic but a strong Republican candidate could capture the seat if Levin leaves.