Sen. Jeff Bingaman (D-N.M.) will retire from the Senate, a Democratic source with knowledge of Bingaman's plans confirmed Friday.

Bingaman is the fourth senator to caucus with the Democrats to announce his retirement this election cycle. The others are Kent Conrad (D-N.D.), Jim Webb (D-Va.) and Joe Lieberman (I-Conn.).

In prepared remarks Friday in Albuquerque, Bingaman said his decision to leave the Senate was not easy. 

"The end of this Congress is the right time for me to step aside and allow someone else to serve," Bingaman said. "There is important work that remains to be done. That is true today, and it will be the case at the end of this Congress. It will be true at the end of every future Congress as well. The simple truth is, there is no ideal time to step aside." 

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National Democrats say they're confident the seat will stay in Democratic control, but Republicans immediately claimed they could flip it to their column in 2012.

As of Friday, the top prospect to replace Bingaman appears to be Rep. Martin HeinrichMartin Trevor HeinrichOvernight Energy: Democrats ask if EPA chief misled on vehicle emissions | Dem senators want NBC debate focused on climate change | 2020 hopeful John Delaney unveils T climate plan Democratic senators want NBC primary debate to focus on climate change Collins offering bill to boost battery research as GOP pushes energy 'innovation' MORE (D-N.M.).

A Democratic source pointed to polling data showing Heinrich leading both former Rep. Heather Wilson (R-.N.M.) and Rep Steve Pearce (R-N.M.). In early January, Wilson indicated interest in running for the Senate.

Bingaman's early announcement is part of a concerted strategy by Democratic Senatorial Campaign Committee Chairwoman Sen. Patty MurrayPatricia (Patty) Lynn MurrayOvernight Health Care: Poll finds most Americans misunderstand scope of 'Medicare for All' | Planned Parenthood chief readies for 2020 | Drugmakers' lawsuit ramps up fight with Trump Overnight Health Care: Poll finds most Americans misunderstand scope of 'Medicare for All' | Planned Parenthood chief readies for 2020 | Drugmakers' lawsuit ramps up fight with Trump Trump's health care focus puts GOP on edge MORE (D-Wash.) to get Democrats to announce their retirement plans early to avoid a situation in which Democrats have to scramble to find a strong candidate. 

The National Republican Senatorial Committee (NRSC) said Bingaman's retirement represents "another strong pickup opportunity" for Senate Republicans. 

"It also further limits the ability of national Democrats to play offense when their resources will be spread out over such an expansive defensive map," NRSC spokesman Brian Walsh said in a statement.

New Mexico Democratic Party Chairman Javier Gonzales said in a press statement that there is a strong selection of potential Democratic candidates to run for Bingaman's seat.

"Moving forward, Democrats in New Mexico have an incredibly strong bench of candidates to draw from and the Democratic Party is going to work hard every day to ensure our nominee wins in November," Gonzales said. 

New Mexico Republicans were brief in responding to Bingaman's retirement. Both New Mexico Gov. Susana Martinez (R) and Pearce praised Bingaman's years of service in press statements.

The departure of Bingaman, the Energy and Natural Resources Committee chairman, will leave a major void in Capitol Hill energy policymaking circles. The moderate shepherded a sweeping energy bill through the Senate and into law in 2007.

It included an array of energy-efficiency provisions and expanded federal programs on carbon capture and storage, among many other measures.

While he’s not seeking another term, Bingaman has a full plate for the remainder of his tenure, including working with the White House on a “clean energy standard."

Ben Geman contributed. 

This post was updated at 3:33 p.m.