Joe Manchin is key to Biden's agenda. Here's why he has so much influence.

Democratic Sen. Joe Manchin of West Virginia is a key vote crucial to President Joe Biden's agenda with Capitol Hill bitterly divided along partisan lines.

Manchin has long been in the spotlight in Washington as a pivotal swing vote unafraid to break with his party over high-profile issues. 

But the focus on the conservative West Virginia Democrat has intensified during the Biden administration with Democrats controlling the narrowest possible majority in the Senate, which means the votes of every lawmaker carry outsized weight.

Facing GOP opposition to key Democratic priorities, many Democrats have intensified calls for the elimination of the filibuster, which sets a 60-vote threshold for most legislation and can be wielded by Republicans to block liberal agenda items.

But Manchin has been a thorn in the side for those who want to get rid of the procedural tool as he has continued to oppose its elimination.

The West Virginia Democrat, who is more of a centrist than many other members of his party, has pushed back on key priorities for Biden.

In a major moment that had a dramatic impact on the Democratic agenda in Congress, Manchin said last year that he was a "no" on the Build Back Better Act, effectively ending negotiations on a broad legislative package under consideration at the time that would have expanded the nation's social safety net. close dialog📷

Manchin had been a key holdout for the legislation, stating concerns over certain provisions of the massive tax and spending bill and how it could exacerbate soaring inflation in theclose dialog📷

Now, Manchin is making headlines once again. On Thursday, the senator dealt a devastating blow to Democrats' hopes for sweeping legislative action this year, telling Senate Majority Leader Chuck Schumer and his staff "unequivocally" that he won't support the climate or tax provisions of a Democratic economic package, two sources familiar with the talks told CNN. close dialog📷

The two had been negotiating for months, and Schumer, a New York Democrat, had made a number of concessions to pare back the climate provisions to appease Manchin, whose support is critical in an evenly decided Senate. close dialog📷

Manchin is open to letting Medicare negotiate prescription drugs prices and to extending enhanced Affordable Care Act subsidies for two years, one of the sources said, which suggests that's all Democrats are likely to get in the package. close dialog📷