Sen. Ben Nelson, D-Neb., is expected to announce later Tuesday that he will not seek a third term in the Senate, according to reports from several news outlets.

Nelson, who has earned both praise and criticism for his center-right positions and his willingness to cross the aisle, has been struggling to decide whether to run for re-election. Democratic leaders have reportedly been urging him to run in order to help the party maintain its slim majority in the Senate, while conservative groups like Crossroads GPS and Americans for Prosperity have been launching attacks in the hopes of discrediting him among Nebraska voters, who tend to lean Republican.

Earlier this month, Nelson told Politico that these attacks only made him more likely to run.

It's like the cat that chases something that's running, he said. There's an urge to say, 'OK, now, let's go.' They run the risk of getting me in. If they want me out, maybe they ought to leave me alone.

But his low poll numbers may have led him to conclude it wasn't worth it to wage a long-shot re-election campaign.

In some ways, Nelson's centrism made him an easy target. He often alienated his fellow Democrats by siding with Republicans on key issues, but he was not conservative enough to gain the trust of most Republicans, leaving him on a shaky middle ground.

The spotlight on Nelson shined the brightest in late 2009, when the Democratic leadership in the Senate made several concessions on the health care reform bill in order to secure his deciding 60th vote. The language of the bill was revised to allow states to prohibit abortion coverage on their insurance exchanges, and the federal government is paying the full cost of expanding Nebraska's Medicaid program.

But while Nelson has often been a thorn in the side of the Democratic leadership, they would much rather have him than lose his seat to the Republicans, which is what now seems likely to happen.