Sen. Jeff Merkley (D-Ore.) has signed on to Sen. Mary Landrieu's (D-La.) bill that seeks to allow people whose insurance plans have been canceled to keep them, becoming the sixth Democratic senator to back the effort.

Merkley has signed on as a co-sponsor, joining Landrieu and Sens. Dianne Feinstein (D-Calif.), Kay Hagan (D-N.C.), Mark Pryor (D-Ark.) and Joe Manchin (D-W.Va.).

All but Feinstein and Manchin face reelection in 2014.

Merkley isn't considered a top GOP target, but he recently drew a challenge from Dr. Monica Wehby, who Republicans hope can put a blue state in play if Obamacare's problems continue into the election year.

Former president Bill Clinton this week also expressed support for letting people keep their insurance, saying the Obama Administration should find a way to keep its promise.

There is also an effort led by House Republicans to allow people to keep their insurance if they want to -- a promise that President Obama made repeatedly -- despite many having had their plans canceled in recent weeks for not meeting the health law's standards. One Democrat, Rep. John Barrow (D-Ga.), has joined the House GOP's effort.

Every Senate Democrat voted in September 2010 against a proposal by Senate Republicans to repeal the portion of the law that allowed for plans to be canceled if they didn't meet certain parameters. Manchin was not in the Senate at the time.