A spokesman for MSNBC said neither Olbermann nor the network would comment beyond the statement he gave Politico.
Chris Hayes, an MSNBC contributor who often fills in for Olbermann, tweeted: "I'm not filling in on Countdown tonight because I didn't feel comfortable doing it given the circumstances."
Sen. Bernie Sanders, I-Vt., also jumped to Olbermann's defense.
"It is outrageous that General Electric/MSNBC would suspend Keith Olbermann for exercising his constitutional rights to contribute to a candidate of his choice. This is a real threat to political discourse in America and will have a chilling impact on every commentator for MSNBC," Sanders said in a statement.
"At a time when the ownership of Fox news contributed millions of dollars to the Republican Party, when a number of Fox commentators are using the network as a launching pad for their presidential campaigns and are raising money right off the air, it is absolutely unacceptable that MSNBC suspended one of the most popular progressive commentators in the country."
Olbermann said he contributed to Grijalva's campaign on Oct. 28, the same day the Democrat appeared on Countdown. Grijalva is opposed to Arizona's tough new anti-immigration law and risked his seat in Congress when he supported a boycott of the state.
According to Politico, Olbermann made the contribution from a Mailboxes Etc., near his show's studio at 30 Rockefeller Plaza in New York City.
Just three weeks before Grijalva appeared on Countdown, Olbermann devoted a portion of his nightly show to attacking News Corp., the parent company of right-leaning cable network Fox News, for contributing to the Republican Governors Association and the US Chamber of Commerce.
Olbermann asked a leading Democratic lawmaker if Congress should pass a law banning journalists from making political contributions.
In an Oct. 7 interview with House Majority Whip Jim Clyburn, D-S.C., Olbermann asked: "Is there a legislative response to the idea that there is a national cable-news outlet that goes beyond having a point of view and actually starts to shill for partisan causes and actually starts to donate to partisan groups of one party?"
According to a 2007, MSNBC.com report, NBC News requires employees there to seek the permission of the network's president before making political donation or participating in political events like rallies.
In September, the watchdog group the Center for Responsive Politics reported that "235 people who identified themselves on government documents as journalists, or as working for news organizations, who together have donated more than $469,900 to federal political candidates, committees and parties during the 2010 election cycle."