LANSING, Mich. — Four Democratic candidates have withdrawn from Michigan’s January 15 presidential primary, undercutting the validity of the contest.

Barack Obama, John Edwards, and Bill Richardson filed paperwork today, the deadline to withdraw from the ballot, a spokeswoman for the Michigan Secretary of State’s office, Kelly Chesney, said.

A fourth candidate, Joe Biden, said in a statement that he was bypassing Michigan’s primary, calling it a beauty contest.

“Today’s decision reaffirms our pledge to respect the primary calendar as established by the DNC and makes it clear that we will not play into the politics of money and Republican machinations that only serve to interfere with the primary calendar,” a Biden campaign manager, Luis Navarro, said.

All of the Democratic candidates already have agreed not to campaign in Michigan because it broke Democratic National Committee rules when it moved its primary ahead of February 5.

Other Democratic candidates had until the end of the day to decide if they’ll stay on the ballot.

Party rules say states cannot hold their 2008 primary contests before February 5, except for Iowa on January 14, Nevada on January 19, New Hampshire on January 22 and South Carolina on January 29.

The calendar was designed to preserve the traditional role that Iowa and New Hampshire have played in selecting the nominee, while adding two states with more racial and geographic diversity to influential early slots.

As punishment, the DNC has vowed to strip Michigan and Florida, which scheduled its contest on January 29, of their delegates.

“We’re very disappointed and this is another example of why the monopoly that Iowa and New Hampshire have needs to end,” a Michigan Democratic Party spokesman, Jason Moon, said.

He declined to speculate about whether the party may decide to move back its primary, or to hold a caucus instead.